There are many types of trusts. They are set up by a donor or settlor who place the assets in the possession of a trustee to look after them for the beneficiaries, the people the donor or settlor intended to eventually give them to.
They are just a legal contract establishing your wishes.
Discretionary trusts are flexible and allow the trustees to do things like change the beneficiaries. Changes can be Limited by the settlor when the trust is established.