i2c routing through FPGA

Hello,

I would please like to ask about AN706:

It states:

"HPS I2C0 is routed through the FPGA interface and acts as a master to various on-board I2C slaves: ...... A bi-directional buffer, ALT_IOBUF, must be added in the design to connect the I2C signals to an external open drain IO. The buffer can be included by instantiating ALT_IOBUF in ghrd_top.v"

I don't understand, what does it mean that " I2C0 is routed through the FPGA" ?

And what is the purpose of ALT_IOBUF ?

Thank you!