A torque/speed curve plots the torque of a motor against the speed that it is going at. The relationship between the torque and the speed is critical to choosing the right motor for your application. Consider the torque/speed curve below for a typical stepper motor.
As you can see the torque at lower rpm is very high but as the speed builds within the motor, the torque begins to drop away. It is important to bear in mind that different motors will have unique torque speed curves and also that with stepper and bldc motors, the controller used will also directly affect the torque speed curve. The key to using torque/speed curves is therefore to understand the exact or minimum requirements that you need from a motor and therefore use the torque speed curve to make a much more accurate assessment of the motor that you need.