A Microsoft file hosting and synchronization service.
The email you received might be fake as there's a version of phishing scam like that going around now. Who's the sender?
A legitimate email from Microsoft regarding your account always comes from domains ending in @accountprotection.microsoft.com
Also, iCloud is not even Microsoft product, it is an Apple product. So that's what also made me think you might be getting phishing scam emails. I've seen those emails, they do reference iCloud sometimes and sometimes they mentioned OneDrive.
As for primary vs. secondary username, Can you sign into your personal account via this link: https://account.live.com?
If so, go to Your Info > Sign in preference. From here you can make comast email primary.