Consider an electron moving left to right in horizontal direction enters a region of a constant magnetic field B which points in the upward direction is acted upon by a force having direction into the plane of the screen which is in accordance to the Fleming's left hand rule.
When the left palm extends thumb, middle finger and the index finger mutually perpendicular to each other then the index finger represents the direction of magnetic field. Middle finger represents the direction of motion of a positive charge (assume reverse if the charge is negative) and the thumb represents the direction of force acting on the charge.
