If you enjoy their Glow Premium services, you will be automatically subscribed to the yearly subscription at the end of the trial period.
If you'd prefer not to subscribe, cancel your free trial at least 24 hours before the end of the trial period by navigating to the sidebar ☰ > Settings. Clicking your Subscription status will open up your Subscription settings in iTunes/Google Play. Tap on the subscription and then toggle off "Automatic Renewal" to cancel.