Slippage is used when the signal is sent to the MetaTrader server.
Slippage defines maximum difference between the price which sent to the MetaTrader sever and actual execution price.
If slippage is too small then Copier will have to try opening orders many times. If slippage is too big then orders on Master and Slave accounts can be opened with big price difference.
Slippage is defined in pips, 1 pip = 0.0001 for non-JPY pairs and 0.01 for JPY pairs. 
To change slippage please open Forex Copier 2 program, click “Settings” on Receiver account, switch to “Advanced settings” tab and change “Slippage:” value.
After this, please click “Save and Finish” and “Apply New Scheme” buttons to apply these new settings.