A mass on a spring is driven by a large geared motor apparatus, and exhibits resonance at the appropriate frequency. Below resonance, the driving motion is in phase with the motion of the mass. At resonance the mass is 90 degrees out of phase with the driving motion, and above resonance it is 180 degrees out of phase. Note the amplitude of motion difference when the system is at resonance.

Driven mechanical oscillation is directly analagous to the oscillation of current in an RLC circuit, where the capacitor is like the spring, the inductor is like the mass, and the resistor is like air friction. Read more about harmonic oscillation here.