Complex sentence: At one particular frequency, these two reactances are equal in magnitude but opposite in sign; that frequency is called the resonant frequency (\\ scriptstyle f_0\\,) for the given circuit.
Split into simpler sentences: At one particular frequency these two reactances are equal in magnitude but opposite in sign. The frequency at which this happens is called the resonant frequency (\\ scriptstyle f_0\\,) for the given circuit.