The graph shows various characteristics of the motor, plotted against torque, for a fixed input voltage of 12 V.
Black is current, blue is speed, green is power output and purple is efficiency.
At the left, there is no torque, so no power output and 0% efficiency because of zero power. The speed is 36000 rpm, and the current is very small.
On the right, the maximum torque is 21.4 mN-m, and that is stall because there is zero speed and maximum current. With zero speed, there is also no power output and 0% efficiency because of zero power.
Maximum power occours at about half maximum torque and half maximum speed. Maximum efficiency is at about 3.5 mNm and 30000 rpm.
If you reduce the voltage, that graph doesn't apply. The maximum speed and maximum torque will both reduce in approximate ratio with the supply voltage, but the peak efficiency won't change much, but it will happen at a lower speed.
If you reduce the voltage for starting, there will be a lot less torque, so it will be slower. Against that, if you halve the start voltage, that will halve the start torque, but it will reduce the power by about 4 times, so you are getting half the spin-up rate for 1/4 the power.