What is a soundpost and what is it used for?

The soundpost in a violin is a dowel (wooden cylindrical rod) which is placed on the inside of the violin between the soundboard (top) and the backplate (bottom)., on the treble side (A and E-string position) just behind the bridge.

As it is placed and held by friction, there is a possibility that it can fall, when all strings are loose for example.
Be sure there is no way you can fix this yourself.
The luthier has a special tool for this.

The placement of the sound post is crucial for the tone of the violin.
Moving it 25mm can make huge differences, so it is advised not to try this yourself.