Amount to mirror allows you to adjust the risk your account will take when auto-copying another user. Amount to mirror measured in % terms.
If you auto-copy another account and set the Amount to Mirror at 100%, your account would copy trades with the SAME % of risk taken by the master Signal. e.g:
If you had £10,000, copying a signal with £1,000 on account. If the Signal placed a trade of 0.01 lots you would copy at 0.1 lots. Your account size is 10x larger - therefore your trade size will be to match the risk.
If you changed your amount to mirror to 200%. Using the same example your lot size would be double the Signals e.g. 0.2 lots. You are therefore INCREASING your exposure.
If you entered 50%, your lot size would be half 0.05 lots. In this instance you are DECREASING your exposure.