A put wall is the strike below current price where put open interest creates the largest concentration of dealer gamma. As price falls toward this strike, dealer gamma forces buying of the underlying to maintain delta-neutrality, producing the mechanical support that gives the level its name.
Put walls hold most reliably in positive-gamma regimes and when the put-side IV trough is deep (a signature of dealer-sold puts — see the IV trough depth term). They break decisively in negative-gamma regimes, where the same gamma that would have caught the move instead amplifies it.
How traders use it
Use the put wall as a long-side reversal target in positive-gamma sessions. Treat a clean violation as a regime-change signal, not a level to fade.