What does stateless design mean in REST?

Stateless design means the server does not store any client session state between requests. Every request from the client must contain all the information needed to understand and process it — the server cannot rely on knowing what the client did in a previous request. Authentication tokens (JWT or API keys) are sent with every request in the Authorization header rather than stored in server-side sessions. This constraint has significant benefits: the server can scale horizontally easily (any server can handle any request since no session affinity is needed), fault tolerance improves (a server restart does not invalidate sessions), and load balancers can distribute requests freely. The tradeoff is larger request payloads since the client must repeatedly send credentials.