Resource efficiency means using the things we have wisely and carefully so that they last longer and we don't waste them.
Let's say you have a big bag of candy. If you eat all the candy really fast, without taking breaks or sharing with your friends, you will soon have no more candy left. But if you eat the candy slowly, make sure to share it with your friends, and save some for later, the candy will last longer and you will have more fun with it.
Resource efficiency is similar to this. Instead of using up all of our resources quickly and not caring about what happens to them, we try to use them carefully and make sure they last a long time. For example, if we use too much paper, we could run out of trees to make new paper. But if we recycle paper and use it again, we can save the trees and the planet.
Being resource efficient means we are being smart and responsible with the things we have, so that we can continue to use them for a long time without running out or harming the environment.