Self-Sovereign identity (SSI) gives users control and ownership over their identity. In centralized and federated identity, a user's identity is an account on a server, and if you delete the account you terminate the user's identity on the internet. With self-sovereign identity, users manage their identity through a digital wallet.
SSI is the end goal of identity 3.0. By using cryptographic keys, ownership of identity is transferred to the individual. SSI means an individual can be an issuer, verifier, and holder of identity. Since identity data is decentralized, no central authority owns the data. The user encrypts their identity documents with their public key, and the verifier can decrypt them on the blockchain where the user's public key sits. The individual holds private keys.