Soak a rag with water and lay it on the affected area. Turn on a household clothing iron to its highest setting (typically cotton). Press the hot iron for 30 – 60 seconds, until the rag begins to dry out a bit. The heat from the iron will force the water from the rag into the fibers of the wood. The wood will expand to its "pre-dented" shape. After ironing, let the wood dry and sand the surface lightly with 100 grit sand paper.
This trick works well on most wood species, prior to applying a finish. It works best on dents that do not tear the wood fibers, but merely stress or distort the fibers.
For additional information please read our wood care and finishing guide