A flat surface on which to dance.
Traditionally dance were held in ballrooms, in which the dance floor was in the center of the room and there was seating around the edges. Dancing progresses counterclockwise/anticlockwise if at all. Dancers performing stationary dances sharing the dance floor with those performing moving dances danced in the center so others could dance around them.
In modern western times the dance floor often refers to the part of a bar, club or similar on which people are dancing or which has been set up for dancing. Some modern facilities have portable dance floors which are basically interlocking wooden sheets put down over the carpet in a conference room to allow for dancing. The main benifit of portable dance floors is that they allow multi uses of the same space.
Generally the best dance floor are sprung wooden floors, because these are the most forgiving on the dancers ankles and shins.