Windmills require straight and steady winds, so their blades usually need to be at least 100 feet off the ground. That’s quite high, and it’s why you don’t see many backyard windmills.
Wind power comes from huge windmills that are connected to the electric grid. The electricity at your house comes off the electric grid, but it really comes from any source. When you buy “100% wind power,” you are ensuring that every unit of electricity you take off the grid is matched by a unit coming onto the grid from some big windmill somewhere.
Also, the matching isn’t happening all the time. It’s usually done on a monthly or even annual basis. That’s how you can get power to your house with wind—even when the wind isn’t blowing.