If you placed the current carrying conductor in the magnetic field, then force acts on the conductor is perpendicular to both direction of magnetic field and current flowing through the conductor.
Now we arrange the thumb, middle finger and index finger of the left hand in orthogonal to each other then, thumb represent the direction of force, middle finger represents the direction of current and the index finger represents the direction of magnetic field.
Reference Source:
[1] Wikipedia Contributors. John Ambrose Fleming. Wikipedia. Published August 9, 2021. Accessed November 30, 2021. https://en.wikipedia.org/wiki/John_Ambrose_Fleming.
[2] Wikipedia Contributors. Fleming’s left-hand rule for motors. Wikipedia. Published October 20, 2021. Accessed November 30, 2021. https://en.wikipedia.org/wiki/Fleming%27s_left-hand_rule_for_motors.