What is a HTTP status code?

The three-digit number a web server answers with: 200 fine, 404 missing, 500 broken, 503 overloaded.

Every page request gets a code. 2xx means success, 3xx a redirect, 4xx a problem with the request (404 not found, 403 forbidden, 429 too many requests), 5xx a problem on the server (500 error, 502 bad gateway, 503 unavailable).

Uptime checks read the code; a 500 on your homepage is an outage even though the server answered.

What to do

A 502 or 503 right after a deploy usually means the application crashed; check the logs.
A 403 on your own site can be a firewall rule blocking the wrong country.
Status code watch alerts when your homepage's code changes.