In this Cryptography method, the sender and receiver uses a common key to encrypt and decrypt the message. Any person with the encryption key can access the message.
It is an abbreviation of the word Synchronize. The word sync means to integrate with a service to get updated. Like sometimes you may have seen Whatsapp asking for the permission to sync with your contacts.! It basically integrates your contact list, so that whenever contacts are updated , Whatsapp will be able to identify that change in the device.