GATT defines the way that two Bluetooth Low Energy devices transfer data back and forth. Uses the generic data protocol called the Attribute Protocol (ATT).
GATT server & client
The peripheral is known as the GATT Server. It holds the ATT lookup data and service and characteristic definitions
The phone/tablet is the GATT Client which sends requests to this server.
All transactions are started by the GATT Client (the peripheral), which receives a response from the GATT Server.