What you?ll need: An iron, some water and a shop towel (an old t-shirt or rag also works!)
Put a dab of water on the affected area, just enough to cover the dent.
Now cover the affected area with your towel/rag.
Set your iron to the highest/hottest setting and make small movements (back and forth and in circles) until towel is dry.
The wood should absorb the water and expand back to its original position. Repeat process as needed.
BAM! For unfinished pieces you can lightly sandpaper the area to smooth things out.