Only threads that are created as 'question' type threads, will have the option to select a best answer. Threads that are created as a 'discussion' type (such as this one), don't have the option to select a best answer. So, it depends on the type of thread.
It's also only an option to the creator of the thread. If you didn't create it, you can't select the best answer.
The email will be referring to a specific thread that you created. That thread must have been created by you, and a question type thread. You can select the best answer, by clicking the best answer button, located on each post.