Where an inductor and capacitor cancel, and what each looks like either side of that point — series and parallel, with the reactance of each at your working frequency.
Where L and C cancel — and what each looks like either side.
Resonance is where an inductor’s and capacitor’s reactances are equal and cancel. This finds that point and shows the behavior around it.
Real components have resistance and stray reactance, so a physical circuit resonates a hair off the ideal and never reaches infinite (or zero) impedance.
