It's a way for a decentralized service to provide a publicly-resolvable self-sovereign identifier for a user. "Publicly-resolvable" means that anyone can query your public identity state, given your DID. "Self-sovereign" means that you and only you can alter your public identity state (i.e. you own the private keys).
A blockchain isn't strictly necessary. You could build a DID method for your PGP key that relies on the world's set of keyservers to resolve your DID's state, for example. You could similarly have DID methods that resolve your public key via Keybase, via DNSSEC, via certificate authorities, and so on. As long as the underlying system is decentralized -- i.e. it spans multiple peer administrative domains -- it doesn't matter whether or not it's a blockchain.
A blockchain isn't strictly necessary. You could build a DID method for your PGP key that relies on the world's set of keyservers to resolve your DID's state, for example. You could similarly have DID methods that resolve your public key via Keybase, via DNSSEC, via certificate authorities, and so on. As long as the underlying system is decentralized -- i.e. it spans multiple peer administrative domains -- it doesn't matter whether or not it's a blockchain.