Those are the intonation adjusters.

The usual setting method is to adjust them to get the note fretted at the 12th fret to match the harmonic at the 12th fret.

Fretting the string causes a slight change in the tension, which shifts the pitch slightly. This effect is more pronounced on some strings than on others, which is why the bridge saddles are not all at the same position. Depending on variables like the string gauge and how firmly you press on the strings, the basic setup may need to be further compensted to get acceptable tuning.