Complex sentence: Of rather more interest from the point of view of filter theory is the behaviour of a resonant circuit when driven by an external AC signal: there is a sudden peak in the circuits response when the driving signal frequency is at the resonant frequency of the circuit.
Split into simpler sentences: Of rather more interest from the point of view of filter theory is the behaviour of a resonant circuit when driven by an external AC signal. There is a sudden peak in the circuits response when the driving signal frequency is at the resonant frequency of the circuit.