Sources add to the ethos of your argument by providing knowledge and perspectives that you don’t have.
An argument usually has a thesis (what you’re claiming), evidence, reasoning (how the evidence connects to the claim), acknowledgement (what someone who doesn’t agree with you would say) and response (how you’d refute that).
Crafting an argument can lead you to change your mind. If you can’t find a response for an acknowledgement, you might have to alter your thesis.
When you summarize sources, you restate their point. When you synthesize, you combine multiple ideas to turn them into something new.