The Received Signal Strength Indicator (RSSI) is a measure of the power level at the receiver. When a device scans for Bluetooth devices, the Bluetooth radio inside the device provides a measurement of the RSSI for each seen device. It's measured in decibels, dBm, on a logarithmic scale and is negative. A more negative number indicates the device is further away. For example, a value of -20 to -30 dBm indicates the device is close while a value of -120 indicates the device is near the limit of detection.

Obtaining Distance from RSSI
Distance Estimation of Smart Device using Bluetooth
Testing if a Beacon is Working
Beacon Location Accuracy
Beacon Signal Stability Observations
Determining Location Using Bluetooth Beacons
Performance Comparison of RSS Algorithms for Indoor Localization in Large Open Environments