Rub a candle against the door and the frame where the two rub together. The wax makes the surfaces smooth enough for the door to slide open easily.
Open the door. Hold the door jamb and pull the door up. If the door shifts, then a hinge is not secured to the jamb properly. Pull the door again and examine the hinges to determine the source of the problem. Tighten the loose screws to move the door back into position.
Check the hinges for a screw in a stripped hole. Remove the hinge covering the stripped hole. Fill the hole with a small piece of wood dipped in wood glue. Replace the screw when the glue is dry.
Examine the door and frame to locate the area where the door rubs. Use a hand plane to remove an eighth of an inch of wood from this section of the door.