Questions: what does it mean to get 507 code from envoy?

*Title*: *what does it mean to get 507 code from envoy?*

*Description*:
Hi, I just get response code: 507 with response text: "exceeded request buffer limit while retrying upstream". And I just cant understand what happend, because the only trace of that error I can find is link to the place where error was raised in source code:
https://github.com/envoyproxy/envoy/blob/main/source/common/router/router.cc#L687
But I still just don't get what exactly happenned there.
Could you please explain it?

*Relevant Links*:
https://github.com/envoyproxy/envoy/blob/main/source/common/router/router.cc#L687

P.S. mb that info should be described in documentation?