Whenever you are trading with CFDs (Contracts for Difference) and you wish to open a position with a given instrument, there will be a required margin of funds in order to open and maintain the position.
The funds will appear as blocked while the position remains open and they will be released again once the position is closed. The blocked funds will change depending on whether your position is profitable or losing and that will respectively affect your account status.
If you are to open a position with 1000 CFD units of Gold at the current price 1,728.10, the margin for opening the position will be £4 680.92. These funds will appear as blocked until the position is closed.