Airtable Apps (modular apps you can add on to your base to bring new information into Airtable, visualize and summarize your records, integrate with other services, and more) are only available on Airtable Pro plans.
If you are part of a workspace on a Pro plan and decide to downgrade from Pro to free or Plus, existing installations of apps in your bases will be disabled. This means that you will be unable to view or interact with these apps.
If, in the future, you decide to re-upgrade your workspace plan from free or Plus to Pro, the disabled apps in your bases will be reenabled with the same settings that they had at the time when you downgraded.
Please bear in mind, though, that if you changed the structures of your bases while your workspace was downgraded and your apps were disabled (by removing views or fields that were referenced by particular apps, for example) that reenabled apps may need to be reconfigured.