A small smart device which allows Bluetooth devices to broadcast or receive tiny and static pieces of data within short distances. In simplistic words, it consists of two parts: a broadcaster (beacon device) and a receiver (smartphone app).
An iBeacon device's signal range is adjustable between a few centimeters and 100 meters. While adjusting the range for your notifications, keep in mind that wider ranges consume more power. Also consider the factors in vicinity that may negatively interfere with the signal range, such as metals, other signals, etc.