The 100 (Continue) status code indicates that the initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and processed.

In cases where the client sends a large request body, it can be inefficient to send the entire body only to have the request rejected due to issues such as invalid headers. To avoid this, clients can send the Expect: 100-continue header to check if the server will accept the request before sending the full body.