A scene is a BDSM encounter between two or more people; it may or may not involve sexual activity.
A scene can take place in private between two people, or in a club where it can be viewed by an audience. When a scene takes place in public it may be because the participants enjoy the thrill of being watched by others, or because of the equipment available, or because having third parties present adds safety for play partners who have only recently met. There is an etiquette in most clubs which regulates how the onlookers may interact with the people in the scene.
It is sometimes used as a verb, meaning to act out a scene.