Contrary to a Trust System, the Trustless setups do not have a central party/authority that the users need to rely on. Trustless applications are also called decentralized as there is no central authority or single point of control.
An example of a trustless setup could be a decentralized exchange ... such exchanges make it possible for users to trade their coins/tokens in a peer-2-peer way, without relying on the exchange to hold their coins and transfer them from one user to the next.