PGP keys in your keyring have to be synchronized with a key server to ensure that any changes made to the key on the key server, like a new signature, are updated in your local copy of the key. Similarly, when you change information on your local key copy, like the expiration date, or when you sign someone's key, you should synchronize the keys to update the changes on the remote copy of the key. You need a working Internet connection to synchronize your keys.