Every object has a natural frequency of vibration. If it is forced to vibrate at this frequency the vibrations will interfere constructively and be larger.
The driving pendulum is made to vibrate. The natural frequency of a pendulum isWhen the driving pendulum is made to vibrate, it does so at it's natural frequency. All of the pendulums on the right have different lengths and different natural frequencies, but one of them has the same length, and so the same natural frequency as the driving pendulum. This pendulum, labelled P in the diagram above, will will vibrate with a far larger frequencies than the others because it is vibrating at it's natural frequency.