Some examples of resonance are a pendulum, a playground swing, a tuning fork and a human voice at a pitch sufficient to shatter glass. Resonance is a vibration system or external force driving another system to oscillate with greater amplitude at a specific preferential frequency.
A pendulum on a clock and pushing a child on a swing requires an initial amount of energy. If the child wants to go higher, or reach a higher amplitude, it requires more energy or a harder push at the right moment for the harder push to have its effect.
Memorex audio cassette tapes aired a series of television ads in the 1970s that featured Ella Fitzgerald singing until she hit a high note that shattered a glass. Her voice, the oscillator, at the right frequency, achieved the volume, or amplitude, powerful enough to destabilize the glass molecules and shatter it. Ultrasound machines used to shatter kidney stones operate according to this principal. The machine oscillations transfer enough energy into the kidney stone for it to vibrate and shatter into smaller, more passable pieces.