You can add secrets (aka environment variables) to your application and to your previews. They are saved encrypted into Coolify's database.
If you would like to change one, you need to stop/start or restart (there is a separate button for it) your application.
Preview Secrets (applications)
Preview secrets are different from normal secrets. They only used for PR deployments. With this, you can setup databases for PR deployments, like a staging environment.