Engine torque is the torque that the engine applies to the drive shaft to turn the propeller.
Propeller torque is the torque that is caused by the rotational drag as the propeller turns. The propeller torque opposes the rotation of the propeller.
In steady state operation, the two are equal. Whenever they are not ("unbalanced torque") then engine speed will change in the direction of unbalance, until they again match. For example, a fixed-pitch prop.