The Unique Node List is a list that contains a set of trusted XRPL validators that a network participant (or XRPL server) trusts to be legitimate. As the name suggests, these entities validate ledger versions by comparing incoming transaction proposals to the last-closed ledger through something called a consensus mechanism (or XRPL’s Consensus Protocol).
If 80% of the validators in the UNL approve of the independently built versions proposed by other participants, they will relay this information to other validators in the list and update the last-closed-ledger upon its validation.
If the server in question is configured to trust the unique node list, this means they believe that at least 80% of the UNL’s validators will be behaving in a way that does not condone any misconduct. In other words, they believe the UNL will not try to defraud them or the network.
UNL validators are usually selected on the basis of their “performance, proven identities, and responsible IT policies” and can, therefore, mostly be trusted to perform their due diligence on the network. If a server wishes to connect to these UNLs, it can refer back to the default lists of high-quality validators published by Ripple, the XRP Ledger Foundation, and Coil. You can view a complete list here.
If a server wishes to connect to other validators, it can do this at will, fully understanding the risk that these validators may not be entirely trustful.