What does Net Gamma mean?

Net Gamma is the sum of call gamma - sum of put gamma 

Why does Net Gamma matter?

This allows you to estimate whether a stock has a larger call gamma position (if figure is positive) or put gamma position (if figure is negative). 

How to use Net Gamma?

A positive gamma position infers lower volatility in the underlying stock. A negative gamma position implies higher volatility in the underlying stock.