The soundpost is the dowel made of spruce that transfers the vibrations from the top of the instrument to the back.
It is located inside the instrument, below the treble foot of the bridge. The soundpost should never be adjusted by anyone other than a trained violinmaker. Too many f-holes and tops have been ruined by musicians with a good ear, but lack of training in this area.
The tops and backs of instruments expand and contract with changes in humidity. Because these pieces are glued onto the sides all around, the height of the arching changes slightly. This can cause the soundpost to be either too tight or too loose, resulting in sound quality variation. If you notice a sudden change in the sound of your instrument, especially after a drastic change in the humidity, don't despair, for it may change back again. You may, however, need to have the soundpost adjusted.
In dry weather, a tight soundpost situation is compounded by the fact that the top and back wood become harder as they dry out. So even if you have a tight soundpost moved inward to release excessive tension, you will not attain the same sound experienced in more humid weather. Depending on the stiffness of the wood, some instruments simply sound better in more humid weather, and some in dryer weather. A Dampit might help.
In extreme cases of low humidity, the soundpost can exert so much pressure on the top and back that they can crack at the contact point with the post when exposed to additional stress. This is a serious situation for the instrument because it is very expensive to repair. Such damage also devalues the instrument significantly.