Send notifications to your phone from your sketch. Learn more from the examples page.
This shield is one of the special shields. Special shields allows your Arduino board to use some capabilities your smartphone can do, like playing music, taking a picture, getting notifications, accessing the clock data and using the touch screen for various functions. Learn more from the special shields tutorial.
Notification library allows your Arduino to send notifications to your smartphone once a certain action occurs.
Adding #define CUSTOM_SETTINGS along with #define INCLUDE_NOTIFICATION_SHIELD is a key that will let you unlock the Notification shield only amongst all shields and prevent 1Sheeld library from eating your memory specially for UNO board.