Bluetooth low energy (BLE) beacons are small, typically battery powered, devices that periodically broadcast an identifier to their surroundings. They are an inexpensive way to make applications aware of their surroundings.
With beacons no user action is required for an application to obtain the proximity information. When a smartphone enters the range of a beacon, it can automatically provide information.
BLE beacons transmit packets of data (callef advertisements) that contain the identity of the beacon for example once every second. In the receiving application it is possible to define a scan interval (how often a bluetooth receiver is turned on) and a scan window (how long the bluetooth receiver stays turned on).
Typically you want to make sure that advertising frequency is set such that there will be at least one advertisement for each scan window.
When a new beacon is observed during a scan, we can say that we have entered its range.
Security issues: Typical beacons are just one-way transmitters that broadcast their identifiers. This means that it is quite easy for a malicious person to replicate the signal. The common solution for this problem is to rotate the identifier of the beacon periodically in a secure way - Eddystone based beacon platform address this issue.