What happens when frequencies match?

Hint: The source of frequency is the vibration. When the vibrations of the sources (objects) match with the external vibrations of the environment or the surrounding, the sources start to vibrate with a higher amplitude and this process is called the resonance.

Complete answer:
The vibrating objects are the source of the sound waves. The examples of the vibrating objects are, such as the vocal cord, the strings of the violin. This vibration caused by the vibrating objects passes through the medium. The other way of creating vibrations is, when the objects fall or pluck, they start vibrating. These examples considered may vibrate with a single frequency or a set of frequencies. Thus, a single frequency or a set of frequencies at which the objects start to vibrate is called the natural frequency or the resonant frequency.
As the environment or the surrounding is a combination of the vibrations (such as sound waves), called as the external vibrations concerning a particular object.
When the natural frequency of the object matches with the external vibrations, the object starts to vibrate with a higher amplitude than earlier. This process is called the resonance.
\[\therefore \]When the frequencies match, a process called the resonance occurs.

Note:
The process called the resonance occurs when the amplitude of any vibrating objects increases when its vibrations match with the vibrations of the other objects. For example, when an opera singer sings in front of a wine glass, the wine glass will remain unbreakable, until there is a match of frequencies of the wine glass and the frequency of the opera singer’s song.