Why In a series RLC circuit the inductive and capacitive reactances are opposite in sign, so at the resonant frequency where XL = XC they cancel completely. What remains in the series path is only the resistance, so Z = R and the impedance reaches its minimum value. That is also why series resonance produces maximum current for a given applied voltage.
Watch out Saying the impedance is high compared to the resistance describes a parallel resonant circuit, where the tank looks like a large resistance at resonance. Matching XL or XC alone ignores the fact that those two reactances cancel each other out.
Series resonance: reactances cancel, only R is left, Z is minimum and current is maximum.