A breach of trust is when a Trustee violates a duty or a responsibility that is given to them in a trust. There are different ways to commit a breach of trust. A breach of trust can occur through fraud, negligence, etc. It is when a promise is broken by a Trustee who gained the responsibility through equity.
Trusts are set up so that a party can have its assets or property managed by separate entities or "Trustees." So, Trustees are "trusted" to manage the assets or property in accordance with the terms of the trust. If a Trustee breaks the terms of the trust, either through direct malicious actions or in some other way, then it is said to be a breach of trust. A breach in trust can result in removal of the Trustee. The person who committed the breach can also be held liable.