Something is reusable if it can be used more than once. Since you can wash a cloth napkin after you use it, it is reusable. Toilet paper? Not reusable.
Many cities now encourage shoppers to bring their own reusable bags to stores, instead of having groceries and other goods put in plastic bags that are usually thrown away after a single use. Reusable is a word you'll often see alongside words like recyclable, renewable, or sustainable, in an environmentally conscious context. It comes from the "again" prefix re- and usable.