What does HTTP status 428 (Precondition Required) mean?

HTTP_STATUS_PRECONDITION_REQUIRED

A server uses 428 to force clients to make a request conditional, typically with If-Match and an ETag. This prevents a stale client from overwriting a newer representation it never read.

Fetch the current resource, retain its validator, and submit the update with the required condition. Do not replace the conditional request with an unconditional write merely to avoid 428.

Servers commonly use 428 to require conditional writes such as If-Match so that one client does not silently overwrite another client's newer version. Fetch the current validator, apply the intended change to the current representation, and send the required precondition rather than issuing an unconditional update.

RFC 6585: Additional HTTP Status Codes