Natural resource depletion is loosely defined as when resources are used up in an area. Non-renewable resources (things like oil and trees) will eventually be depleted while renewable resources (things like wind and sun) typically are not entirely depleted.
Resource depletion is a couple of really big words for a very simple idea. It's the idea that raw materials are resources needed for something get used up in a particular area. So non-renewable sources all of those will eventually be used up since they don't renew themselves. Well renewable resources may or may not get depleted depending on the resource and how you use it. Some examples of resources that may one day getting depleted could include oil, ultimately the amount of oil that we have on this planet is relatively limited. There's perhaps some ways of making it but ultimately if we keep pumping it out of the ground we'll ultimately deplete ourselves off all the oil. Trees, they can grow. Trees make more trees but if you chopped down all the trees in an area, you chop down so many that they cannot replace themselves faster than you chopped them down then you may wind up with resource depletion. Even things like helium, people don't really think of helium as a vital resource but I remember hearing some article, some thing on the news where they're talking about US may run out of the helium they need, so in case you're worried about it you may not be able to do your squeaky voice anymore in the future.