The gamma of an option position is the rate of change of the delta of the position with respect to the asset price.
For example, a gamma of 0.1 would indicate that, when the asset price increases by a certain small amount, delta increases by 0.1 of this amount.
wHEN THE GAMMA OF AN OPTION WRITER'S POSITION IS LARGE and negative and the delta is zero, the option writer will lose significant amounts of money if there is large movement (either an increase or a decrease) in the asset price