Moving through a series of exercises with little or no rest between them, completing one round before repeating the sequence.
A circuit is a giant set for the whole body: goblet squat, push-up, row, plank, kettlebell swing, 40 seconds each, one minute between rounds, three rounds. Twenty minutes, and the heart rate never comes down.
It trains work capacity and muscular endurance well, and it fits small-group sessions, because six people can start at different stations. What it does not train is maximal strength, since the loads have to stay light enough to move under fatigue.
The usual mistake is using it as the whole program. A client who only ever circuits gets fitter for a few months and then plateaus, because nothing in the session ever gets meaningfully heavier. Use it as a finisher or as one session a week beside real strength work.