Breakout rooms are smaller sessions split off from a larger meeting so groups can work in parallel before regrouping. The term now covers both the virtual feature in Zoom, Teams and Meet, and the physical rooms a workshop spills into.
Physical breakouts are a planning item: a 20-person workshop might need one large room plus three huddle rooms for the same time slot. Booking them together, rather than hoping nearby rooms are free, is the difference between a smooth session and a corridor scavenger hunt.