command line "root=/dev/mtdblock0" 's function???

I do not understand these two command line "root=/dev/mtdblock0" and the command line "root=/dev/mtdblock0 rw rootfstype=jffs2" 's function everyone when make menuconfig add one of them in the configurations.Help me and explain these two command 's function when add them, one by one is better !

THX!!! http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/ph34r.gif