A successor trustee is someone assigned to manage and take care of one's assets after they are no longer able to do so (most commonly due to death or serious illness). The person creating the trust is referred to as a grantor, and in legal terms they act as their own trustee while they're still managing their assets. A legal trust is very different from a traditional will/inheritance, as it imposes some strict limitations over what the trustee is allowed to do with the assets they're receiving. In most cases, the grantor will have outlined a very specific course of action which the trustee is expected to follow.
Authorities are responsible for overseeing the execution of a grantor's trust properly. Sometimes, the successor trustee will be allowed to personally benefit from the grantor's assets and operations, when they have been named as a beneficiary as well as a trustee. However, if they aren't, they're obliged to utilize the assets in a way that would have benefited the grantor normally, and they're usually asked to invest any returns in the company they're taking care of.
A trust becomes finalized and cannot be revoked after the moment it becomes active - that is, after the grantor becomes unable to perform their duties. The trustee is not allowed to modify the trust in any way, and is legally obliged to follow through with any guidelines set in the trust. In cases of disagreement or unwillingness to execute the trust, severe legal consequences may be imposed over the trustee.