What is "mark best response" in discussions?

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So I have searched mylot to try to find out what "mark best response" is when you start a discussion and someone gives a response. I see it's there but not sure what it's for or why you should do it? If some one could let me know that would be great. Sorry if this is explained some place on mylot. I'm using my iPhone to do mylot and it's hard to find and see things that are on a small screen. Thank you for the help.