Dealer hedging
The stock that dealers buy or sell to keep their option books neutral as prices move. On a GEX chart, a long bar means a lot of it is tied to that strike.
Dealers who sold or bought options at a strike hold stock or futures to stay neutral. How much they must buy or sell as the price moves depends on those options' gamma times their open interest. When a GEX chart shows a long bar at a strike, it means a large share of that rebalancing is tied to options at that strike.
Whether the hedging leans against moves or follows them depends on the sign. Where dealers are long gamma, they sell as the price rises and buy as it falls. Where they are short gamma, they buy as it rises and sell as it falls. Both depend on the model's assumption about which side dealers hold.
Illustrative example (hypothetical numbers, not a real trade): a strike carries 20,000 calls of open interest with a gamma of 0.03. That is 20,000 × 0.03 × 100 = 60,000 shares of hedging per $1 move. If dealers are long those calls, a $1 rise means selling about 60,000 shares and a $1 fall means buying them.