To reuse something is to make use of it more than once, or for a subsequent time. Asking for a plastic bag at the grocery store might seem wasteful, but you can reuse that bag many times.
Instead of throwing something away after using it once, ask yourself if there's any way you can reuse it. You can reuse a paper towel roll by putting it in your hamster's cage so he can crawl through and chew on it. You can reuse containers like glass jars and plastic tubs by storing leftovers in them. You can even reuse kitchen scraps by composting them and using the compost in your garden.