A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood.
It is an OPTIONS request using two HTTP request headers: Access-Control-Request-Method and Access-Control-Request-Headers, and the Origin header.
A preflight request is automatically issued by a browser when needed; in normal cases, front-end developers don't need to craft such requests themselves.
For example, a client might be asking a server if it would allow a DELETE request, before actually sending a DELETE request by using a preflight request:
OPTIONS /resource/foo Access-Control-Request-Method: DELETE Access-Control-Request-Headers: origin, x-requested-with Origin: https://foo.bar.org
If the server allows it, then it will respond to the preflight request with a Access-Control-Allow-Methods response header that lists DELETE:
HTTP/1.1 200 OK Content-Length: 0 Connection: keep-alive Access-Control-Allow-Origin: https://foo.bar.org Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE Access-Control-Max-Age: 86400