This article discusses the Unique Node List (UNL) and why it is so important to the XRP Ledger.
The UNL is a the list of validators a given participant believes will not conspire to defraud them. Each server operator can choose their own UNL with which they want to cooperate, usually based on a default set provided by a trusted publisher.
Since anyone can run a validator node, the burden is on the participants of the network (XRPL) to choose a reliable set. Right now, three publishers (Ripple, Coil, and the XRP Ledger foundation) are known to publish recommended lists of high quality validators, based on past performance, proven identities, and responsibility towards the system. However, every network participant can choose which validators it chooses as reliable, the list provided by the three publishers are mere recommendations towards the server operators.