If you've asked a question, you'll have the option to mark any reply as the Best Answer. Marking something as Best Answer gives the question a green checkmark to show others the question has been solved.
Best Answers also help the author of the answer to rank up in the community. They're a nice way to say "Thanks" to the person who took the time to post. 😄
To mark a post as Best Answer, just scroll to the bottom of a post and click "Mark as Best Answer."