An object free to vibrate . It vibrate at a specific rate called the object's natural, or resonant, frequency. The rate of the vibration i.e how many vibration in one second or frequency depends on the size, shape, and composition of the object. Vibration in object will strong when it is vibrating at a frequency equal to or nearly equal its natural frequency. This phenomenon is called resonance.Through resonance, a comparatively weak vibration in one object can cause a strong vibration in another.
let we have two functions
y=sin(x) and y=sin(.99x),y=sin(.95x)
see graph of y=sin(x) and y=sin(.99x) ,and y=sin(.95x) , functions have nearly equal frequency,shown in red ,gree and blue .These functions are showing resonance.First two function are nearly close while third is not very close.