On the Pro Trial, the top workspace in your account is upgraded to a complimentary Pro plan for 14 days. You can select which plan to move the workspace to after the Pro trial by clicking the Pro trial Banner to the right of the workspace's name.
If you would just like to compare the Pro trial with the free plan, you can create a new free workspace by clicking Add a workspace below the Pro trial workspace.
If your workspace is on a Pro trial and you let the trial expire, your workspace will lose access to the premium features only available on the Airtable Pro plan. These include apps, premium forms, advanced calendar features, record coloring, personal and locked views, domain/password restricted share and invite links, year-long snapshot and revision history, and increased record and attachment limits (50,000 records per base versus 1,200 on free, 20GB of attachments per base versus 2GB on free).