WebFinger is a discovery protocol that lets software ask a domain where information about an account or other resource can be found. In the Fediverse, it is commonly the step that turns an address such as @alice@example.social into the ActivityPub actor URL that another server can fetch.
From @name@domain to an actor URL
A remote server sends an HTTPS request to the target domain's /.well-known/webfinger endpoint, usually with a resource parameter such as acct:alice@example.social. The response is a JRD, or JSON Resource Descriptor, containing links associated with that account. An ActivityPub implementation can advertise the actor document with an application/activity+json or application/ld+json link. The querying server then retrieves that actor document and learns the actor's inbox, outbox, public key information and other properties needed for federation. WebFinger therefore does not carry posts or follows itself; it resolves a human-friendly identifier into machine-readable locations.
Why the Fediverse needs discovery
Without a discovery layer, users would have to exchange long actor URLs instead of addresses that look roughly like email addresses. WebFinger gives federated software a domain-controlled, standard way to perform that first lookup. It is also older and broader than ActivityPub, which is why the same mechanism can appear in other decentralised identity and discovery systems. For administrators, the endpoint is small but important: if it is broken, remote servers can fail to find users even when the ActivityPub actor itself is healthy.
What WebFinger does not do
WebFinger does not prove that an account is trustworthy, and it is not a global directory. A client must already know the domain and resource it wants to query. Fediverse software also varies in exactly which links and media types it expects. The protocol itself is standardised in RFC 7033; ActivityPub deployments use it as a practical convention rather than because the ActivityPub Recommendation mandates one particular account-discovery protocol.