RFID reader antennas emit electromagnetic radiation (radio waves).

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

If it is more than the distance of one full wavelength away, it is said to be in the "far field."

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

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