Think of Credo like an academic Wikipedia. Like Wikipedia, it is focused on background research and serves as a tertiary source. Unlike Wikipedia, it pulls from a collection of professionally made Encyclopedias from around the world and offers you discrete entries from each.
Odds are, you are already familiar with Wikipedia. You may even chafe a bit when professors ask you not to use it. But what they are generally asking is that you don't stop at Wikipedia, not that you don't use it at all. This page will give you some information on how to use Wikipedia properly in an academic context.
In Wikipedia's own article, "Academic Use," they give a good rundown of the strengths and weaknesses of the platform from an academic point-of-view. I recommend you check it out if you want to see Wikipedia's own reasoning for why you shouldn't use it as an academic source.