Torque

Torque is a physical value in classical mechanics. The measuring unit for torque is Newtonmeter (Nm).
(Source: http://de.wikipedia.org/wiki/Drehmoment)

A high clamping force is required to guarantee a tight screw connection between two parts. In order to obtain this clamping force the screw is tightened until a certain torque is reached. Apart from torque, other factors like depth of insertion and rotation angle are taken into account to ensure the quality of the screw connection.