A breakout is price pushing decisively through a previously established support or resistance zone. "Decisively" usually means a close beyond the level, ideally on increased volume or after a period of tight consolidation. Traders buy breakouts of resistance and sell breakdowns of support, targeting follow-through momentum.
Most breakouts fail. The best filter is patience: wait for a close beyond the level, then wait for a retest — price returning to the broken level and respecting it as new support or resistance — before entering. This sacrifices some of the initial move but dramatically improves the win rate.