What is BLE?
BLE stands for Bluetooth Low Energy, which is a wireless computer network technology. Compare with “Classic Bluetooth”, BLE reduces power consumption and lower cost, while maintaining a similar communication range.[1]

Bluetooth SIG defines many profiles for BLE, where a profile is a specification for how a device works in a particular application.Manufacturers are expected to implement the appropriate specification(s) for their device in order to ensure compatibility.

The GATT profile is the general specification for sending and receiving short pieces of data. All the current BLE application profiles are based on GATT.

Some developer may want the distance between 2 devices with BLE module. There is not possible to get precise distance, instead there is a RSSI(Received signal strength indication), which is a measurement of the power present in a received radio signal.
Which iOS device support BLE communication?
iPhone 4S and later
iPad 3 and later
iPod Touch 5
iPad mini
How to start?
It is better to start from official document of Core Bluetooth Programming Guide[2] from Apple.
And the sample code of BLE transfer from Apple. In the code, it provides good example to transfer long text from peripheral and central nodes.

Reference:
[1] Bluetooth low energy
[2] Core Bluetooth Programming Guide
[3] sample code of BLE transfer