@Snoochenpurner Hello, I'm not on the Challenges Team but I can offer an answer.
CAS stands for Create-A-Sim. You know when you start a new save? The first thing you have to do is create your sim(s), either an individual sim or a household.
CAS is that place where you design them, give them traits/aspirations, pick their outfits, etc.
Once you're done, you can save the household to your MyLibrary by clicking the folder icon in the top right, then click the check mark at the bottom right to start playing (choosing where to live).