Radio Frequency IDentification (RFID) tag is a device for the administration of a huge number of products at a very low cost. The operation of this device is started when enough energy was captured to send the signal back. Even if there is sufficient powerful signal, capturing enough ambient RF energy from it to power even the smallest device is a good trick, though it can be done.
The hurdle it is needed to clear for ambient RF harvesting is the Friis equation: