Gamma is the second derivative of an option's price with respect to the underlying — mathematically, the rate at which delta changes as the underlying moves. A position with high gamma sees its directional exposure shift rapidly as price moves; a low-gamma position is more stable.
Long options have positive gamma: deltas accelerate in your favor as the move continues. Short options have negative gamma: deltas accelerate *against* you, which is why short-options traders blow up during fast moves. The textbook example is selling near-the-money puts before earnings — small profit if nothing happens, catastrophic loss if the underlying gaps 10% overnight, because the negative gamma turns a small position into a fully-leveraged short equity exposure in one print.
Gamma is highest for at-the-money options near expiration. Selling 0DTE (zero days to expiration) at-the-money options is the highest-gamma exposure available in retail markets, which is exactly why it pays the highest premium and exactly why it has destroyed the most accounts in the post-2021 retail options boom.
Practical risk management for options traders includes a gamma limit alongside the more familiar dollar-loss limit. Most professional vol desks size positions to a max acceptable gamma exposure as a percentage of the book, then dynamically hedge with the underlying to neutralize it. Retail traders almost never do this — they trade options as directional leveraged bets, then discover gamma the hard way during the first overnight gap that runs against the position.