Flash can be used in one of tow ways: either as your main source of illumination, or as a fill in to give a boost to the shadows.
If you are using flash as your main source of illumination (as distinct from fill flash), then the flash will stop the motion for you. If you are using it as a fill, then it wont freeze motion.
When using flash as you main (or only) illumination source, it will freeze most motion because the duration of the flash is MUCH shorter than your shutter. A typical flash speed is about 1/10,000th of a second (give or take).
If you are using flash as your main illumination, and you are outside, then you are also using a small enough aperture and a shutter speed such that ambient light alone would under expose your subject by several stops.
So, even though your shutter is open for say 1/60th of a second, this duration has little to no impact on the exposure of your subject. Thus your subject is only illuminated for 1/10,000th of a second, so thats your effective shutter speed from the point of view of stopping motion.
Thus the flash is the primary subject illumination. It may not have an effect on the illumination of the background though, depending on how far away the background is. So be prepared for dark backgrounds.
Flash is a complex topic, particularly when combined with ambient light, thats will be better addressed in its own course, so in this lesson I just want you to be aware that flash can be used to freeze motion.