This function gets called by libcurl after a connection has been established or a connection has been reused (including any SSL handshaking), but before any request is actually made on the connection. For example, for HTTP, this callback is called once a connection has been established to the server, but before a GET/HEAD/POST/etc request has been sent.
See: https://curl.se/libcurl/c/CURLOPT_PREREQFUNCTION.html
This can be used, for example, to make sure that connections/requests to the local network are blocked.