Provides factual information and background material on the subject at hand or establish why the subject is a problem that needs addressing.
For an example situation, after introducing the fact that Emily is wearing some horrific, God-awful hat, your narration could go on to say that it's awful, you and hopefully her, both know that it's awful, it's hurting your eyes, and it needs disposed of immediately.
Includes the proof needed to make the writer's case
Claims and evidence to back up your thesis!
i.e. Explain that it's not normal for her to do that; Emily has a fairly dependable fashion sense and is usually a reliable resource for all things fashion; she once told YOU that your boots were horrendous and that they looked like her foot, make sure to support your claim!