A CORS preflight request is a special http request that checks to see if the CORS protocol is understood by your backend http server.

It is an OPTIONS request, using three (3) HTTP request headers:

1. Access-Control-Request-Method
2. Access-Control-Request-Headers
3. Origin

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, an http client might be asking your http server backend if it would allow a DELETE request, before sending a DELETE request, by using a preflight request

credits: https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request

All Posts