There are 3 values that describe the power of an Estimote beacon's signal: Broadcasting Power, RSSI and Measured Power. Read more about the differences between them
Broadcasting Power is the power with which the beacon broadcasts its signal, i.e. the power with which the signal leaves the beacon's antenna. The owner of the beacon can change this setting e.g. using the Estimote app. The value ranges between -30 dBm and +4 dBm, lowest to highest power settings respectively. The higher the power, the bigger the beacon's range and the more stable the signal, but it also shortens the battery life of the beacon—so you might want to consider lowering it to the minimum required by your use case.
RSSI is the strength of the beacon's signal as seen on the receiving device, e.g. a smartphone. In general, the greater the distance between the device and the beacon, the lesser the strength of the received signal. Think: if you drop a stone into a container with water, the wave is bigger near the point of impact and smaller further away.
This inverse relation between the distance and RSSI is used to estimate the approximate distance between the device and the beacon using another value defined by the iBeacon standard: Measured Power (see below).
Note that, due to external factors which influence the Bluetooth radio wave broadcasted by beacons—such as absorption, interference or diffraction—the RSSI value tends to fluctuate. The further away the device is from the beacon, the more unstable the RSSI readings will be. And, since distance approximation is based on RSSI, this directly translates to less accurate estimates at greater distances.
Measured Power is a factory-calibrated, read-only constant which indicates what's the expected RSSI at a distance of 1 meter to the beacon. Combined with RSSI, this allows to estimate the actual distance between the device and the beacon.