A Microsoft file hosting and synchronization service.
Mary,
Looks like the warning is about a different account.
Looks like you have one account with a Microsoft 365 subscription, which gives you the 1TB storage. And the problematic account doesn't have any subscription, hence it is only at 5GB, and you have exceeded that allowance.
And no, data will not be automatically transferred.
On the account you currently use, can you go to https://account.live.com > subscriptions, and tell me, what's the plan you have? Is it Personal, Family or Premium? I need this information to determine what options you might have to transfer the files.