Gamma is the rate of change of an option's delta for a $1 move in the underlying asset. To illustrate, imagine a 2000 strike call option has a 50 delta (0.50) with the spot price of the underlying equal to $2000. If the spot price increases by $1, the option will increase in value by $0.50, and the delta will change too. Imagine the delta increases to 52 (0.52). The 0.02 change in delta is approximately equal to the gamma of the option.