Depends on the damage.
If you just want to restain ... those citrus paint strippers work well. Coat the floor with them, then let sit overnight, and remove.
If they are small nicks/scratches ... It could just be a matter of sanding and restaining/sealing the floor.
If they are deeper ... You may have to use a planer, rather than a sander.
If the boards are rotted ... you'll have to cut out the area and replace.