Important Note: No Link to MTA
This does not directly link to your MetroCards. The MTA does not provide a way to link a physical card to their data so this is impossible at this time.
What this app does is allow you to easily keep track of your cards' balance(s) on your own so that you'll always know how much is left on your card and aren't caught at the turnstile with an insufficient fare.
Free vs Pro Version
The Free version allows you to keep track of only one MetroCard and contains ads. With the Pro version you get:
no more ads
ability to store and track multiple MetroCards
card expiry notifications: enable notifications for when your card will expire 30 days, 7 days, one day before, and/or the day of expiration
home screen widgets: reduce the balance of a MetroCard by a single fare with a simple click, without having to open the app