If a wall, a door, or a room is panelled, it is covered or decorated with flat pieces of wood fitted together. The wood is not just painted on; it sits on the surface in squares or long boards, often with thin frames around each piece. You hear this about old dining rooms, courtrooms, and ships: a panelled room feels warm, heavy, and rather formal. Though actually, people now use it for other materials too — a leather-panelled door, for example — but the first picture in a learner’s book is still wood. It is mainly a British spelling; American books usually write paneled, with one l. Same word, same meaning.