An audience is an intended group of people that a text is appealing to. An audience can be as broad as "adults" to as specific as "residents of Cobb County, Georgia". Audience is important to a writer because the writer's style differs depending on who they are writing to or for.
Who is the intended audience for The Atlantic (and why)?
The intended audience for The Atlantic is older teens or adults primarily located in the United States. The news stories and articles written by The Atlantic authors and contributors are often about political or social matters that only a mature teenager or established adult could fully comprehend. A child or younger teenager would lack the mental maturity to grasp what any particular passage was discussing. Most articles on The Atlantic are geared towards United States residents, as they are often about United States politics (specifically, Donald Trump and his recent policies and/or speeches) and are written in American English. The complex and often negative/neutral behavior of these articles, passages, or publications are tailored so that an average (American) adult would be able to understand the gist of the writing, while an average ten year old would find it difficult to know "What Perpetrators of Mass Violence Learn From Each Other" (an article written recently on The Atlantic).