The 12x100mm thru-axle is the standard front axle on most modern gravel bikes. The numbers simply refer to the axle diameter (12mm) and the hub spacing (100mm). It threads directly through the hub and into the fork, creating a stiff, secure connection that keeps your brake rotor aligned consistently — which matters more than you'd think when you're descending loose gravel at speed.
The big advantage of 12x100mm is wheel compatibility. Since it's the shared standard across gravel and road bikes, you'll have the widest selection of wheelsets to choose from if you want a second set for road riding or a replacement. You can swap between most gravel and road wheels without worrying about fit.
Just don't confuse it with 15x100mm, which is a mountain bike standard found on some gravel bikes with suspension forks — those two are not interchangeable. If you're buying a standard gravel bike without suspension, 12x100mm is what you'll almost certainly get, and that's exactly what you want.