Torque is a measure of the ability to twist something so the torque required to turn a propellor (or helicopter rotor) is a measure of the effort being put into moving the air with the prop and so the effort in moving the aircraft forwards (or upwards). I've avoided words like thrust and power because they aren't the same thing - the rate of producing torque (ie Newton metres per second) is in fact power as torque shares the same units as work.
One of the confusions regarding torque is the units it is apparently measured in on some aircraft. Torque's real units are ft pounds or Newton metres. However, at least one PT-6 powered aircraft I've flown has its torque gauge calibrated in PSI! The reason for this is that the device for measuring torque in many engines relys on twisting a shaft with a helical gear on the end. The helical gear makes the shaft push into its bearing which is oil filled and so it compresses the oil and this pressure has a direct relation to torque. To be honest, the gauge could just as well be calibrated in cabbages as long as you know how many cabbages is max torque etc.