It is a command to silence auctions and it came with the plugin, so the creator of the plugin made that as one of the silence commands for the FloAuction plugin.
It's part of the plugin, it's just an alternative of /auc quiet EDIT: from the plugin's dev page: floAuction - In-game auction software ... (v 2.1+) * Per user silencing of auctions: / auction [stfu|quiet|off|silent|silence].