A sticking door is one that, when opening or closing, catches on the floor, the frame or the strike plate at some point. You can tell by the noise, by marks on the paintwork or the floor and because you have to push or lift the door to close it.
The most common causes are loose or worn hinges, a dropped door, a frame that has moved or timber that swells with damp, something very common on the coast in the humid months.
Do not plane it down straight away: if the problem is the hinges, adjusting them makes it close properly again and no wood needs to come off. It is worth fixing soon, because the rubbing strains the lock. More under door repair and adjustment.