If you're safe, you're out of harm's way, or protected from being hurt. These days most parents strap their small children into car seats to keep them safe.
When you protect a person or an object from danger, you keep them safe. Eggs are sold in special cartons so they're safe from cracking, and drivers wear seat belts to keep them safe from being injured in accidents. Cash is kept safe in the bank. In baseball, a player is safe when they touch a base. You can also use safe as a noun, for a special box that keeps money and other valuable items safe.