Need to undestand SD_Card.h

Hi, so I'm just a newbie and I plan to use the SD card of my Altera DE2 board for a project. I managed to find the example that comes with the CD named as DE2_SD_Card_Audio. So I saw that there's a header file named "SD_Card.h". I understand this is the one responsible for reading files that's inside the SD Card. Can someone help me understand the codes in this header file?

#ifndef   __SD_Card_H__
# define   __SD_Card_H__
//-------------------------------------------------------------------------
//  SD Card Set I/O Direction
# define SD_CMD_IN   IOWR(SD_CMD_BASE, 1, 0)
# define SD_CMD_OUT  IOWR(SD_CMD_BASE, 1, 1)
# define SD_DAT_IN   IOWR(SD_DAT_BASE, 1, 0)
# define SD_DAT_OUT  IOWR(SD_DAT_BASE, 1, 1)
//  SD Card Output High/Low
# define SD_CMD_LOW  IOWR(SD_CMD_BASE, 0, 0)
# define SD_CMD_HIGH IOWR(SD_CMD_BASE, 0, 1)
# define SD_DAT_LOW  IOWR(SD_DAT_BASE, 0, 0)
# define SD_DAT_HIGH IOWR(SD_DAT_BASE, 0, 1)
# define SD_CLK_LOW  IOWR(SD_CLK_BASE, 0, 0)
# define SD_CLK_HIGH IOWR(SD_CLK_BASE, 0, 1)
//  SD Card Input Read
# define SD_TEST_CMD IORD(SD_CMD_BASE, 0)
# define SD_TEST_DAT IORD(SD_DAT_BASE, 0)
//-------------------------------------------------------------------------
# define BYTE    unsigned char
# define UINT16  unsigned int
# define UINT32  unsigned long
//-------------------------------------------------------------------------
void Ncr(void);
void Ncc(void);
BYTE response_R(BYTE);
BYTE send_cmd(BYTE *);
BYTE SD_read_lba(BYTE *,UINT32,UINT32);
BYTE SD_card_init(void);
//-------------------------------------------------------------------------
BYTE read_status;
BYTE response_buffer;
BYTE RCA;
BYTE cmd_buffer;
const BYTE cmd0   = {0x40,0x00,0x00,0x00,0x00};
const BYTE cmd55  = {0x77,0x00,0x00,0x00,0x00};
const BYTE cmd2   = {0x42,0x00,0x00,0x00,0x00};
const BYTE cmd3   = {0x43,0x00,0x00,0x00,0x00};
const BYTE cmd7   = {0x47,0x00,0x00,0x00,0x00};
const BYTE cmd9   = {0x49,0x00,0x00,0x00,0x00};
const BYTE cmd16  = {0x50,0x00,0x00,0x02,0x00};
const BYTE cmd17  = {0x51,0x00,0x00,0x00,0x00};
const BYTE acmd6  = {0x46,0x00,0x00,0x00,0x02};
const BYTE acmd41 = {0x69,0x0f,0xf0,0x00,0x00};
const BYTE acmd51 = {0x73,0x00,0x00,0x00,0x00};
//-------------------------------------------------------------------------
void Ncr(void)
{
  SD_CMD_IN;
  SD_CLK_LOW;
  SD_CLK_HIGH;
  SD_CLK_LOW;
  SD_CLK_HIGH;
} 
//-------------------------------------------------------------------------
void Ncc(void)
{
  int i;
  for(i=0;i<8;i++)
  {
    SD_CLK_LOW;
    SD_CLK_HIGH;
  }
}
//-------------------------------------------------------------------------
BYTE SD_card_init(void)
{
    BYTE x,y;
    SD_CMD_OUT;
    SD_DAT_IN;
    SD_CLK_HIGH;
    SD_CMD_HIGH;
    SD_DAT_LOW;
    read_status=0;
    for(x=0;x<40;x++)
    Ncr();
    for(x=0;x<5;x++)
    cmd_buffer=cmd0;
    y = send_cmd(cmd_buffer);
    do
    {
      for(x=0;x<40;x++);
      Ncc();
      for(x=0;x<5;x++)
      cmd_buffer=cmd55;
      y = send_cmd(cmd_buffer);
      Ncr();
      if(response_R(1)>1) //response too long or crc error
      return 1;
      Ncc();
      for(x=0;x<5;x++)
      cmd_buffer=acmd41;
      y = send_cmd(cmd_buffer);
      Ncr();      
    } while(response_R(3)==1);
    Ncc();
    for(x=0;x<5;x++)
    cmd_buffer=cmd2;
    y = send_cmd(cmd_buffer);
    Ncr();
    if(response_R(2)>1)
    return 1;
    Ncc();
    for(x=0;x<5;x++)
    cmd_buffer=cmd3;
    y = send_cmd(cmd_buffer);
    Ncr();
    if(response_R(6)>1)
    return 1;         
    RCA=response_buffer;
    RCA=response_buffer;
    Ncc();
    for(x=0;x<5;x++)
    cmd_buffer=cmd9;
    cmd_buffer = RCA;
    cmd_buffer = RCA;  
    y = send_cmd(cmd_buffer);
    Ncr();
    if(response_R(2)>1)
    return 1; 
    Ncc();
    for(x=0;x<5;x++)
    cmd_buffer=cmd7;
    cmd_buffer = RCA;
    cmd_buffer = RCA;
    y = send_cmd(cmd_buffer);
    Ncr();
    if(response_R(1)>1)
    return 1; 
    Ncc();
    for(x=0;x<5;x++)
    cmd_buffer=cmd16;
    y = send_cmd(cmd_buffer);  
    Ncr();
    if(response_R(1)>1)
    return 1;
    read_status =1; //sd card ready
    return 0;
}
//-------------------------------------------------------------------------
BYTE SD_read_lba(BYTE *buff,UINT32 lba,UINT32 seccnt)
{
  BYTE c=0;
  UINT32  i,j;
  lba+=101;
  for(j=0;j<seccnt;j++)
  {
    {
      Ncc();
      cmd_buffer = cmd17;
      cmd_buffer = (lba>>15)&0xff;
      cmd_buffer = (lba>>7)&0xff;
      cmd_buffer = (lba<<1)&0xff;
      cmd_buffer = 0;
      lba++;
      send_cmd(cmd_buffer); 
      Ncr();
    } 
    while(1)
    {
      SD_CLK_LOW;
      SD_CLK_HIGH;
      if(!(SD_TEST_DAT))
      break;
    }
    for(i=0;i<512;i++)
    {
      BYTE j;
      for(j=0;j<8;j++)
      {
        SD_CLK_LOW;
        SD_CLK_HIGH;
        c <<= 1; 
        if(SD_TEST_DAT)
        c |= 0x01;
      } 
      *buff=c;
      buff++;
    } 
    for(i=0; i<16; i++)
    {
        SD_CLK_LOW;
        SD_CLK_HIGH;
    }
  }
  read_status = 1;  //SD data next in
  return 0;
}
//-------------------------------------------------------------------------
BYTE response_R(BYTE s)
{
  BYTE a=0,b=0,c=0,r=0,crc=0;
  BYTE i,j=6,k;
  while(1)
  {
    SD_CLK_LOW;
    SD_CLK_HIGH;
    if(!(SD_TEST_CMD))
    break;
    if(crc++ >100)
    return 2;
  } 
  crc =0;
  if(s == 2)
  j = 17;
  for(k=0; k<j; k++)
  {
    c = 0;
    if(k > 0)                      //for crc culcar
    b = response_buffer;    
    for(i=0; i<8; i++)
    {
      SD_CLK_LOW;
      if(a > 0)
      c <<= 1; 
      else
      i++; 
      a++; 
      SD_CLK_HIGH;
      if(SD_TEST_CMD)
      c |= 0x01;
      if(k > 0)
      {
        crc <<= 1;
        if((crc ^ b) & 0x80)
        crc ^= 0x09;
        b <<= 1;
        crc &= 0x7f;
      }
    }
    if(s==3)
    { 
      if( k==1 &&(!(c&0x80)))
      r=1;
    }
    response_buffer = c;
  }
  if(s==1 || s==6)
  {
    if(c != ((crc<<1)+1))
    r=2;
  } 
  return r; 
}
//-------------------------------------------------------------------------
BYTE send_cmd(BYTE *in)
{
  int i,j;
  BYTE b,crc=0;
  SD_CMD_OUT;
  for(i=0; i < 5; i++)
  {
    b = in;
    for(j=0; j<8; j++)
    {
      SD_CLK_LOW;
      if(b&0x80)
      SD_CMD_HIGH;
      else
      SD_CMD_LOW; 
      crc <<= 1;
      SD_CLK_HIGH;
      if((crc ^ b) & 0x80)
      crc ^= 0x09;
      b<<=1;
    } 
    crc &= 0x7f; 
  }  
  crc =((crc<<1)|0x01);
  b = crc; 
  for(j=0; j<8; j++)
  {
    SD_CLK_LOW;
    if(crc&0x80)
    SD_CMD_HIGH;
    else
    SD_CMD_LOW; 
    SD_CLK_HIGH;
    crc<<=1;
  }    
  return b;   
}
//-------------------------------------------------------------------------
# endif