A Successor Trustee is someone that will administer your Trust once you pass. You're able to name this person (or people) in your Trust document.
You can name anyone you'd like as your Successor Trustee (including multiple people at once), as long as they are 18 years or older. You can name the same person as a Beneficiary and/or Guardian, although it is not required. You can opt for an attorney or professional Trustee, although also not required.