Okay, let's say you have some fancy math things called "rings" (they're not the kind you put on your finger).
The "approximation property" is when you can get really close to a number in your ring by using other numbers that are really close to the one you want.
It's like trying to hit a target with a dart. If you throw the dart really close to the bullseye, you can still get some points even if you didn't hit it exactly.
In math terms, it means that you can use numbers from your ring - either bigger or smaller - to get really close to the number you want. And sometimes, even if you can't get exactly to the number you want, you can still get really close and that's good enough.