I-485 background checks

[BIN2002]

I need some detail info on background checks done to process I-485
What process do they follow to do these checks ?
The background checks are done in India if she is citizen of india?
Do they goto police stations to enquire our background ?
I didn't understood exactly what does it mean by background check

Could anyone of you explain in detail..
I read almost all threads on this topic but i didn't gain knowledge what exactly they check and where do they check, either Homeland country or the place where we reside ??

Thanks,
Bindu