A call wall is the strike above current price where call open interest creates the largest concentration of dealer gamma. When price moves up toward this strike, the calls' delta increases — and dealers who are short those calls must sell the underlying to remain delta-neutral. That mechanical selling acts as resistance.
Call walls work best when they sit at a strike that is already a high-volume, high-OI level, when implied volatility is not collapsing (which would drag delta down), and when overall market regime is positive-gamma (so dealers are actually hedging in this direction).
How traders use it
Treat the call wall as a fade target in pinning regimes and a breakout level in trending regimes. A close meaningfully above the call wall in a low-gamma regime is one of the cleanest momentum signals on the board.