How to repair the bottom of an interior door greatly depends upon what the damage is that needs to be repaired.
Door Rubbing on Floor
If your door is rubbing on your flooring or carpet it is an easy repair. Close the door and watch to see exactly where it is rubbing and mark this area with a pencil or something that can be washed off. If the door is rubbing on carpet, lay a straight edge on the carpet against the closed door so you can mark the line of where the carpet fits. After you have marked your door, take it off the hinges and use a wood plane to plane down the wood to the markings. Reattach the door and it should not rub.
Damage to the Wood
If the wood is deeply scratched, perhaps from pet damage, it is an easy fix also. Buy a good wood putty from your local home improvement store. Some people also swear by the bonding agent that is used to fix automobiles. Apply into the scratches and cut with a putty knife and wipe away as much excess as possible. Use your putty knife to level it smooth and allow it to dry. When dry, sand smooth, prime the area and then paint the door.