Complex sentence: Different transmitters could then be "tuned" to transmit on different frequencies, and a receiver could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter.
Split into simpler sentences: Different transmitters could then be "tuned" to transmit on different frequencies. The receiver would also have a resonant circuit (tuned circuit), and could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter.