Deactivating the plugin hides its widgets from the frontend but leaves all settings and cached data intact. Reactivating restores everything without needing to reconnect your Google Analytics property.
Deleting (uninstalling) the plugin removes the plugin files from your server. Your settings and cached data stored in the WordPress database are retained unless you manually delete them. Your site remains connected on the GATopPosts.com platform, so you can reinstall and re-enter your API key to pick up right where you left off.