What does "pound for pound" mean?

Posted by SomeDudeOnRedit@reddit | AskAnAmerican | View on Reddit | 131 comments

I hear this all the time, "Pound for pound, she is the best bakery chef in America."

"Pound for pound, Alaskan mosquitoes are the worst."

Is this a boxing reference I don't get? Is it just a way to add emphasis?