gamma is the derivative of how much option values change in relation to underlying stock price.
Gamma is the rate of change of delta w.r.t the underlying price. For example OTM calls have a delta close to 0. As the underlying price increases the delta will increase. When the underlying price reaches the call strike price (ATM) the delta will typically be 0.5. As the underlying price continues to rise and the call becomes deep ITM the delta will approach 1.0.