what is meant by resonance of an LCR circuit ?

Resonance in an AC circuit containing a resistor (R), an inductor (L) and a capacitor (C) is defined as the state when the inductive impedance (ωL) is equal to the capacitive impedance (1/ωC). And the reactance of the circuit is equal to the resistance R. 

This happens at a certain frequency called the resonant frequence. At this frequency the circuit behaves like a purely resistive circuit.