How to interpret the APIcast error log?

Issue

We see the following error in APIcast error log. What does it mean and what are the values that appear in it?

2018/11/02 12:38:45 [error] 31#31: *27988 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 192.168.123.212, server: _, request: "GET /api/v1/products HTTP/1.1", upstream: "https://172.16.1.2:443/api/v1/products", host: "api.example.com"