Someone recently commented on a question I posted "Please accept this answer so it doesn't add back to the queue"? How do I do this? I already gave a up vote for the answer.
There is a check mark next to the answer. Select it.
See How do I accept an answer? for details.
At any given time you can only have accepted one answer per question (you can change which one though).
Tangentially, the concern about "add back to the queue" is usually not a real one, tough. As soon as an answer has positive score it is removed from the unanswered queue. Nevertheless it is good form to accept an answer if there is one that solves your problem. (If not then do not accept.)