Gamma

What does gamma measure?
Gamma is the rate of change of delta, or the derivative of delta. If delta is the first derivative of the option price, gamma is the second derivative of the option price is useful for the measuring the stability of an option's probability of success.

Expiration and Gamma
A high gamma implies a high rate of change of delta and therefore a high rate of change of the option's price. In order to ensure a successful outcome, as methodical traders we hope to maintain a high probability of success. Yet, with high gammas, the location of our option price is unpredictable. 

How to Avoid High Gammas: Rolling Options
As expiration approaches, there is less time for the option price to change. This causes for more fluctuation in delta as the contract approaches expiration, and therefore a higher gamma. In these instances, the options can be rolled in order to restore a longer DTE and therefore lower gamma.