What happens to my files if I cancel the monthly subscription to iCloud plus?

iCloud+ subscriptions increase the space you have on Apple's servers for your devices to access, store, interact and synchronise. If you currently use more than 5GB, which is the basic and free account, then you will lose access to your data and iCloud probably not sync your devices. Eventually Apple will delete your content, though they've never said how long this will take but others have reported at least a few months grace between cancelling an iCloud+ plan and the loss of data.

If you are confident that your use of iCloud space is well under 5GB there will be no problem.

Read carefully APple's notes in Downgrade or cancel your iCloud+ plan - Apple Support

especially:

Before you downgrade or cancel your iCloud+ plan, first download or remove content that exceeds your new storage amount. To make sure that you don't lose any information, learn how to copy what you store in iCloud.

and

What happens when you downgrade or cancel iCloud+

• If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information. iCloud updates again when you increase your storage capacity or delete information to make more space. 
• If you downgrade to the free iCloud plan, you will no longer have access to iCloud+ features like Hide My Email, Private Relay, and HomeKit Secure Video support.
• If you pay for both Apple One and an iCloud+ plan, you can downgrade or cancel the iCloud+ plan and just use the iCloud+ plan included in Apple One. After you select Change Storage Plan, there might be additional steps before you can downgrade or cancel your storage.