I have been keep hearing that gamma is a bet on realized volatility. That is, if we are long gamma then we need higher realized volatility to coming in the future. Otherwise what happens ?

from other source :

If you are long gamma and delta-neutral, you make money from big moves in the underlying, and lose money if there are only small moves.

I get it that why short gamma hurts, but honestly I dont know why long gamma and delta neutral makes money if realized vol is higher.

Please help me and advance thanks!