LIMIT 1; The Integer part only

Hello List;

I have two questions:

1) When writing the function (I mean sql function) in the postgresql, I noticed the use for LIMIT 1, but did not understand what does it mean and why we use it?

2) Also in the function (the sql function) in the postgresql, if I need to take the integer part of the number, which math function can do this for me?

For example, I have 3.900 and I need only the 3 (the integer part), which math function to be used?

Any advise?
Regards
Bilal