What are f-holes and what are they for?

F-holes are sound holes, openings, in the top plate of the violin.

Sound holes have different shapes, depending on the instrument.
Guitars have round sound holes, lutes have rosettes…

The sound hole of the violin is f-shaped, hence its name and are placed symmetrically.

Although the sound comes from the vibration of the body (the surfaces of the plates), the sound holes let the air inside travel outwards.
This way the sound holes help project the sound more efficiently.

There are little notches in the centre of the f-holes, these are used to determine the correct placement of the bridge.