'Decentralized Identifiers' is a new World Wide Web Confortium (W3C) specification. " Decentralized Identifiers (DIDs) are a new type of identifier for verifiable, 'self-sovereign' digital identity. DIDs are fully under the control of the DID subject, independent from any centralized registry, identity provider, or certificate authority." This is a technical document, so for most readers it's only useful to read Section 1.
A decentralized ID is basically a URL that points to some data about yourself. This data includes cryptographic material, authentication suites, and service endpoints. It's worth noting that the DID specification "has produced two specific requirements for a new type of URL" which "SHOULD NOT require a centralized authority to register, resolve, update, or revoke the identifier." We will get to what that might look like in the next module, on Resources.