Current is the rate of flow of charge. In a metal wire the charge that flows is carried by electrons. The larger the amount of charge passing a point each second, the larger the current. Current is measured in amperes, A.
Two conditions are needed for charge to flow. There must be a source of potential difference, such as a cell or battery, which pushes the charge round. There must also be a closed circuit, which is a complete loop with no gaps.
If a switch is opened it leaves a gap, so the circuit is no longer complete and the current falls to zero. The source of potential difference is still there, but the charge cannot flow.
