Useful means helpful or serving a good purpose. A useful tool makes a job easier; useful advice helps you solve a problem; useful information gives you what you need to know.

Think about the difference between owning something and actually getting value from it. You might have twenty pencils in your desk, but they're only useful when you need to write something. A flashlight is useful during a power outage but sits quietly in a drawer the rest of the time. The most useful things are the ones you reach for again and again.

Something useful doesn't have to be fancy or expensive. A sturdy cardboard box is incredibly useful when you're moving or organizing. A simple bookmark is more useful than the most beautiful book cover if it helps you remember your place. Sometimes the most useful person in a group project isn't the one with the best ideas but the one who keeps everyone organized and on track.