Torque and the lever

Torque is a measure of a force's ability to rotate a body about an axis:

$M=F\cdot r$

where $F$ is the force and $r$ — the lever arm, that is, the perpendicular distance from the axis of rotation to the line of action of the force. The unit is the newton-metre (N·m).

A lever is in balance when the torques on both sides cancel out:

$F_1\cdot r_1=F_2\cdot r_2$

This is the principle of the lever — a small force on a long arm balances a large force on a short one (hence "give me a place to stand and I will move the Earth").