The tension wrench, also known as a torsion wrench, is used to apply torque to the plug of a lock in order to hold any picked pins in place. Once all pins are picked, the tension wrench is then used to turn the plug and open the lock.
It is typically shaped like the letter "L", the vertical part elongated to allow you to apply varying levels of pressure, and easily hold while you manipulate the lock pins with a lock pick.
Tension is usually the key to opening all locks. Too much pressure can inhibit you manipulating the lock pins successfully.