The first person you name to manage your trust. For a trust you set up during your lifetime, it is very common to name yourself as the initial trustee of your trust. When you set up a trust in your Will, that trust will not take effect until you pass away, so the trustee will commonly be the same person you name as the executor of your Will.
For example, if you appoint your sibling to manage a trust for your children if you pass away, your sibling is the initial trustee.