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To sustain something means to keep it in existence.
When we talk about resources, sustainability means living today in a way that will let people meet their needs in the future. For example, look at paper and paper products. They are usually made from trees. Sure, new trees can be grown. But if people use up so much paper that we cut down trees faster than new ones can grow, then our use of this resource is not sustainable. Also, it takes a lot of water and fuel to turn trees into paper—and then to get that paper to you.
Today, trees are not in danger of being used up, but other resources are. We have a long way to go before we get to sustainability, but change is happening.
