Wikipedia:Open proxy detection/Explanation

When an IP address has performed an edit, a database with open proxy addresses is automatically searched. This database is periodically maintained and consists of more than half a million IP addresses and IP ranges. If a match is found, it is reported on this page.

This automatic reporting only takes place if the server of RonaldB is online. This is close to 24/7, but there may be downtime from time-to-time.

The system receives input from the irc-stream of recent changes. If the system is operational, it is logged in with the name: op-nnnnn, in which nnnnn is a 5-digit number.

Look here for more (technical) background info.