A Microsoft file hosting and synchronization service.
Thanks for the additional information.
Since Microsoft 365 Basic includes 100 GB of OneDrive storage rather than 1 TB, the 1 TB storage you're seeing is likely being provided by a different entitlement, possibly your Microsoft 365 Business Standard subscription.
Based on that, canceling the Apple-purchased Microsoft 365 Basic subscription may not affect your current 1 TB storage allocation. However, I cannot verify your account configuration from the community forum.
Before making any changes, I recommend reviewing your storage details here: https://onedrive.live.com/managestorage (Use another device or internet source if you still get any error message when loading the page)
Also, after canceling, check that your available OneDrive and Outlook storage still meets your needs before the subscription fully expires.
Since the Microsoft 365 Basic subscription was purchased through Apple, the cancellation must be managed through the Apple App Store subscription settings rather than through Microsoft. If you have questions about billing, refunds, or charges related to that subscription, Apple Support would be the appropriate support channel.