It means that a step (a single pulse from the computer) will move .9 degrees, or 1.8 degrees when instructed to.
All else being equal, including the number of steps per second generated by the computer, the .9 degree will rotate at half the speed of the 1.8 degree. The .9 will divide up the motion into smaller bits though, so the results with a sufficiently precise machine, would be more accurate.
Another way to think of these is 200 steps per revolution, and 400 steps per revolution. So, with a fine pitched screw, let's say 1/4" x 20tpi, the distance moved per step would be .00025" for the 1.8 degree motor, and .000125" for the .9 degree motor.
I'll let you decide if the difference is worth it.
-- Chuck Knight