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What does a reviewer do?

This question is regarding math journals but I assume should apply to journals in other disciplines as well. Some journals ask you to choose a reviewer from their list (or sometimes they ask you to suggest your reviewer of choice) as you are about to submit your manuscript. My question is what exactly this reviewer does to the manuscript? Does (s)he looks at it and decides whether to send it to a referee? or (s)he is indeed the referee?