Imagine that a man carrying a gun runs into your classroom. What is your reaction? Now imagine that man carrying a gun is wearing a police uniform. Do you have a different reaction?
That story is how I explain what context is and why it is important. Put simply, context is anything outside of the text that is needed to make sense of it. Before you know the man is wearing a police uniform, you see gun and read threat. After you see the uniform, your interpretation changes. The police uniform is the context for determining the danger in the situation.