When current flows through a conducting wire which is inside an external magnetic field, the wire (and the external magnet)experience a force. The force on the wire is perpendicular both to the external field and to the direction of the current flow (in other words they are mutually perpendicular) . A left hand can used, as shown in the diagram below,  to represent the three direction all at right angles to each other, using the thumb, fore finger and middle finger. Each finger is labelled with a quantity, these are force, magnetic field and electric current (conventional flow not electron flow).

Fleming's Left Hand Rule

This is quite hard to see and understand from a brief explanation and a diagram. For a quick but full explanation, watch the video below.

Explaining Fleming’s Left Hand Rule