If a node of yours is "considered" by an upper-level monk and enough people have down-voted it without a certain number of upvotes, the node is automatically "reaped". The "user" that does this auto-reaping is the Nodereaper. It is no different from a user on a Unix system that just runs daemons, like 'adm'. Other times, your node might be reaped by one of the Janitors (monks who have been entrusted with a certain set of responsabilities).