That's 25KHz (25,000 x per second). So the voltage is fluctuating between 0V and 12V (square wave means changing voltage is very fast). The duty cycle means how long the square wave is happening as a percentage. If it's 50% then half the time the voltage is a flat ZERO volts. The rest of the time it's the cycling square wave.
Now, again, if you apply a simple 12 Volts, DC, with no pulsing I'm pretty sure it would just spin at 100%.