A transition is the moment between stable positions — the movement from half guard to side control, from turtle to back control, or from guard to top in a scramble. Most of BJJ's dynamic action happens in transitions rather than in static positions.
Transitions create vulnerability for both players. The top player, mid-pass, may overcommit and lose their balance. The bottom player, mid-escape, may expose their back. Understanding the key moments in a transition — when to accelerate, when to stall — is a major part of intermediate and advanced BJJ.
For beginners, transitions are often the hardest part of live rolling: you know what to do in guard and you know what to do in side control, but the chaotic bit in between is uncharted territory. Drilling transitions slowly and deliberately builds the map.