Soundcheck happens before the audience arrives. Each performer plays or sings while the sound engineer sets levels, EQ, monitoring, and effects for every input channel.
A typical soundcheck order: drums first (kick, snare, toms, overheads), then bass, then keys/guitars, then vocals. Each element is checked individually (line check), then together as a full band.
Soundcheck is also when the monitor engineer sets up each performer's personal mix — what they hear in their wedges or in-ear monitors on stage. A good soundcheck is essential for a good show.