Resonance occurs in AC circuits when reactance is effectively eliminated. This can only happen at a specific frequency for any given inductor and capacitor combination. When the inductor and capacitor are in series, it occurs when their reactances are equal and effectively makes the combined impedance 0 Ohms.
This video shows how to determine the frequency at which resonance occurs and then shows the effect on the voltages across components as well as the real, reactive and apparent power in the system.

For more on resonance and AC circuits, check out the free, online, open source textbook here:
https://www.electronx.ca/education/ac...