What is an Expect header?
Indicates that the client requires specific server behaviour before it sends the request body, most commonly Expect: 100-continue.
Expect: 100-continue lets a client check with a website's server before sending a potentially large request body, so it doesn't waste bandwidth uploading data the server was always going to reject (say, for a size limit or missing auth). Support for it varies across HTTP libraries and load balancers, so it's a reasonable thing to explicitly exercise if your monitored endpoints accept large uploads and you want confidence the whole chain handles it correctly.
Common use case
A large file upload client sends Expect: 100-continue first, so the server can reject an oversized or unauthenticated upload with a 4xx before the client wastes bandwidth sending the whole file.
Example
Expect: 100-continueHistory
Part of the original HTTP/1.1 specification, RFC 2068 (1997).
Did you know?
Some HTTP libraries and load balancers implement it inconsistently or not at all, which is why many HTTP clients default to not sending it unless a developer explicitly opts in.