Complex sentence: The advantage of an antenna at resonance is that it presents an impedance to the feedline to the transmitter or receiver which is a pure resistance, while off resonance it presents a reactance, as it has capacitance or inductance.
Split into simpler sentences: The advantage of an antenna at resonance is that it presents an impedance to a feedline connected to a transmitter or receiver which is a pure resistance. When off resonance, it presents a reactance, as it then exhibits capacitance or inductance.