When a HTTP client makes a connection to a HTTPS server, it uses TLS to create a secure connection. TLS can have a bit of a complicated ‘handshake’ to establish the connection. Because there’s a bunch of back and forward, this can take a long time, especially when there’s a lot of latency between server and client.

There are ways for clients to optimize this setup by sending a bunch of data very early in the process, before the full TLS connection is completely setup.

In some cases this can cause security problems. In those cases a the server can tell the client to retry a specific HTTP request after the TLS connection has been fully set up

In these situations, it will return the 425 Too Early status code.

Normally, you would never have to deal with this status code as a developer, unless you are creating HTTP(s) servers from scratch.

References