The electrical definition of a low frequency RFID antenna:
Inductance - loop antenna
Capacitance - tuner box
Resistance - wire size, stranding, material
The inductance and capacitance determine the frequency of operation. The HDX tuning indicator is used to show when the inductance and capacitance are tuned to 134.2 kHz.
Besides tuning an antenna to frequency, the power level needs to be controlled to have a Q that is not too high or low. Q is determined by the the ratio of the inductance to resistance. If the Q is too high (resistance too low and/or inductance too high) then the power levels will be excessive and could damage the reader electronics. If the Q is to low (resistance too hight and/or inductance too low) then the tag detection range will be reduced.