Netting in forex is a system where you can only have ONE position per currency pair at a time.
If you open a new trade in the same pair, it combines (nets) with your existing trade instead of opening a separate one.
Example
You buy EUR/USD with:
0.10 lot
Then later you buy again:
0.20 lot
With netting:
These merge into one buy position of 0.30 lots.
If you sell EUR/USD with 0.10 lots:
It reduces your position instead of opening a new one.