Rethoric refers to a technique in which the speaker tries to persuadethe reader to cnsidering a topic from a different perspective.
In this excerpt from paragraph 5, President Obama uses rethoric to convice his readers that all lives matter.
By saying that Trayvon Martin could have been him 35 years ago, he tries to express that is imposible to know what great things people will do in their future. He wants to highlight that life is precious and should not be taken away so easily.