Normally, an answer to a question can only accepted by the asker. This is because while votes show how many people found an answer useful, acceptance is a sign from the asker that "This is the answer that solved my problem." In a Team, you should always encourage your members to accept answers when they have solved the problem they asked about.
However, there can exist a situation where a question is very important to the team, but the user who asked it is no longer with the organization or has otherwise had their account deactivated. For this scenario, we provide Admins of a Team the ability to accept an answer, in order to help point future readers in the team to the correct solution.
When viewing answers to a question that was asked by a user who is now deactivated, an Admin will see a special "ADMIN" checkmark, which provides the following explanation when moused over:
By clicking this checkmark, the answer can be marked as accepted as if the asker had done so. As with standard acceptance, it can be undone by clicking it again, or by picking a different answer to accept.