There are some addons out there that will allow people to lie about achievements they have. This is designed to help you prevent that.

What this does is scan every nearby party or raid member for a given achievement, and produce a list of everyone with their names coloured accordingly. Green means they have the achievement, red means they don't. White means they were out of range for the scan, or couldn't be scanned for some other reason.

To start a scan, type /truth [Achievement link] (shift click the achievement to insert the link). This will clear any previous scan and start a new one. To re-scan the same query (and keep the previous results), type /truth without any arguments.

Note that only when you start a new query will the list of players be reset. Re-scanning will keep the list, which means anyone who's left the party or raid will remain, and anyone who's joined will not be in. This won't break the addon, but should be noted.

Also note that with a large raid group, the scan will take a few seconds. This is because there is a throttle on the rate at which this information can be retrieved. There is a progress bar that will show you how far along in the list the scan is. The game will usually be slow and choppy during a scan, so don't run one during a boss fight or any other time you need to be able to react.