You can either write a comment to thank ( or probably ask for more details), or choose the answer as the best answer.
If you do not select it as the best answer, the question will be remained as an open question. It means people can still answer your question. After one week, the answer with the highest vote will be automatically shown as the best answer.
If you are satisfied with the answer you received you may select it as the best answer. However, no one else can write any more answer to your question.
Hope it helps :)