A door that sticks, drags, or won't latch is usually a small adjustment away from working again. Find where it's binding before you reach for a plane.
Find the rub
Open and close the door slowly and watch where it touches the frame, or slide a piece of paper around the gap to find the tight spot. Seasonal humidity, settling, and loose hinges are the usual causes.
Try the easy fixes first
Often a door sags because hinge screws have loosened — tighten them, and replace a stripped screw with a longer one that bites into the framing. This alone fixes many sticking doors by pulling the door back into alignment.
Plane as a last resort
If the door still rubs, lightly sand or plane the binding edge, removing just a little at a time, then seal the bare wood so it doesn't absorb moisture and swell again. If the latch misses, adjust or move the strike plate.