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
understandwhat 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
mathfunction 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