Direct impingement means that the gases generated from firing a bullet are redirected through a gas tube. The gas tube feeds back into the upper receiver at the top of the bolt carrier group. When the gas pushes against the bolt carrier group, it causes it to eject the spent casing in the chamber and then feed the next round from the magazine.  Other terms for this are “gas gun”, “DI”, and “canon”.  If you genuinely didn’t know that, I would highly recommend checking out this “How an AR-15 Works” video animation. It’s AWESOME. Also, if you’re brand new to the AR-15 world, I would recommend checking out Palmetto State Armory. You can get a complete rifle for less than the price of this upper receiver. That’s not to say this is a ripoff, it’s just more of a high-end upper, whereas PSA is more entry-level.