Consider your LCR series circuit as the driveN system and the power supply as the driveR.
The driveR (power supply) will make the current in the driveN system (LCR) oscillate at the frequency of the driveR and these are called forced oscillations.
For a fixed amplitude voltage power supply the response (current) in the driveN system depends on the frequency of the driveR.

At one particular frequency the response of the driveN system (current) is a maximum this is called resonance and occurs at the resonant frequency.
At the resonant frequency the circuit behaves as though it is only a resistor, the impedance of the circuit is equal to the resistance of the resistor, and so the current in the circuit is the supply voltage divided by the resistance of the resistor.
Because it is a series circuit the currents through each of the components are in phase with one another but the voltages across them are not, with the voltage across the inductor leading the current by $90^\circ$, the voltage across the resistor in phase with the current and the voltage across the capacitor lagging the current by $90^\circ$.
At resonance the voltages across the inductor and the capacitor are equal in magnitude but exactly $180^\circ$ out of phase with one another.
The impedance of the inductor $\omega L$ is equal to the impedance of the capacitor $\frac{1}{\omega C}$.
Thus the supply voltage equals the voltage across the resistor.
There are many instances in electronics where resonance matters in use and one example is a simple radio receiver called the crystal set.

The radio signal is picked up by an aerial and fed into a tuned circuit comprising an inductor and a capacitor in parallel.
If the frequency of the radio signal (driveR) is the same as the the resonant frequency of the tuned circuit (driveN) the voltage at this frequency across capacitor is very large whereas all the other frequency signals picked up by the aerial produce very much smaller voltages across the capacitor and so are undetected by the rest of the circuit which convert the radio signal into sound.
So the tuned circuit selects the signal from the radio station you wish to listen to and rejects all the other signals.
The resonant frequency of the tuned circuit in this circuit is controlled by altering the capacitance of the capacitor.