A harbor is a safe place providing refuge and comfort. If you're traveling, the harbor provided by a warm hotel is welcome. For ships, a harbor is a sheltered port area shielded from waves, where it's safe to dock.
Harbor can also be used as a verb, which describes maintaining a belief or a feeling. If you harbor ill-will toward your neighbor John, you don't like him much. Harbor can also mean you hold back your ideas and don't express them openly. John may have no idea you hate him if you harbor your true feelings deep inside, but pretend you like him to his face.