Whenever we use a string or rope to exert a force on an object, we're creating tension in the rope that transmits the force we exert at one end of the rope to the object at the other end. This force is usually labeled T or F_T.

We usually assume that the rope has no mass, and does not stretch. When we exert a certain force on our massless unstretchable rope, the rope exerts that same force on the object. Tension makes the rope feel like it's being pulled apart.

One rule to remember - you can't push with a rope. The tension force always goes along a string or rope away from the object attached to it.