I see this question is in the music category. If you mean the musical term; bass, bass is the term used to decribe an instrument. So if you have a Bass clarinet, you have a Clarinet that plays low. It's also used to describe the lowest male voice.
In maths it means the number we group bigger numbers by: normal numbers are grouped by tens, so we say base 10, whereas computers use base 2 numbers (binary).
The base of a geometric figure is the bottom part.
A base in chemistry is a substance which reacts with an acid to make a salt and water only, or in more advanced theories a proton acceptor or electron pair donor.