These are commonly known as "saddles".
Basically, they are used for intonation; bad intonation can lead to some notes sounding a little "off", flat or sharp,
especially when playing harmonics.
A good intonation does the opposite, and will make all the notes on your bass sound exact.
If you go on over to the set up forum and do a search, you'll be able to find plenty of good walk-thrus oh how to intonate!
good luck.
