What do you understand by an HTTP OPTIONS request?

HTTP OPTIONS requests are used to obtain information about the available methods and capabilities of a resource before actually making a request. It allows the client to determine the set of supported methods (GET, POST, PUT, DELETE, etc.) and other options such as headers that can be used when accessing the resource.

Typical use of OPTIONS: CORS (Cross-Origin Resource Sharing) preflight request. The browser sends an OPTIONS request to the server before making a "complex" request (for example, with the PUT or DELETE method, or with custom headers) to a cross-domain resource. The server responds to the OPTIONS request indicating whether such a request is permitted.

The server's response to an OPTIONS request contains an Allow header listing the supported methods. The Access-Control-Allow-Methods header in the context of CORS indicates which methods are permitted for cross-domain requests.

OPTIONS /resource HTTP/1.1  
Host: example.com  
Origin: http://another-domain.com  
Access-Control-Request-Method: POST  
Access-Control-Request-Headers: Content-Type, Authorization  

HTTP/1.1 200 OK  
Allow: GET, POST, HEAD, OPTIONS  
Access-Control-Allow-Origin: *  
Access-Control-Allow-Methods: POST, GET, OPTIONS  
Access-Control-Allow-Headers: Content-Type, Authorization  
Access-Control-Max-Age: 86400