RFID reader antennas emit electromagnetic radiation (radio waves).

If an RFID tag is outside of one full wavelength of the reader, it is said to be in the "far field." If it is within one full wavelength away, it is said to be in the "near field."

The far field signal decays as the square of the distance from the antenna, while the near field signal decays as the cube of distance from the antenna.

So passive RFID systems that rely on far field communications (typically UHF and microwave systems) have a longer read range than those that use near field communications (typically low- and high-frequency systems).