Okay kiddo, a waxed jacket is like a coat that has been covered in a special kind of wax. This makes the jacket waterproof so that you don't get wet when it rains.
Think of it like cookies that your mom bakes. Before she puts them in the oven, she might put a little bit of butter or oil on them so they don't stick to the baking tray. In the same way, this special wax is put on the jacket so water doesn't stick to it.
Waxed jackets were first made for people who worked outside, like farmers and fishermen, because they needed to stay dry while they worked. But now, lots of people wear them for fashion too because they look cool!