At first, the Catalog application should be added to your Jira. Read how to find, add and manage the Cloud apps (Atlassian documentation).
You can trial Catalog free for at least 30 days. Once the trial period is over, the cost of the app will be automatically included in your next bill for monthly cloud users, unless manually deactivated. Check the Atlassian FAQ dedicated to marketplace apps for more information.
What will happen if unsubscribed?
The Catalog sections (user section, issue panel) will keep visible yet the information won't be available. The Catalog custom fields will stay available. When subscribed again, the information from these fields will be synchronized to Catalog in case of changes.
What will happen with data if my license is stopped?
The Catalog section, custom fields and settings will not be available. However, the data itself will stay safe so you will easily proceed with the app after renewing the subscription.