What is meant by the fundamental frequency?

For example, if the fundamental frequency is 50 Hz (also known as the first harmonic) then the second harmonic will be 100 Hz (50 * 2 = 100 Hz), the third harmonic will be 150 Hz (50 * 3 = 150 Hz), and so on.

The lowest frequency produced by any particular instrument is known as the fundamental frequency. The fundamental frequency is also called the first harmonic of the instrument.

The fundamental frequency (n = 1) is ν = v/2l . The higher frequencies, called harmonics or overtones, are multiples of the fundamental. It is customary to refer to the fundamental as the first harmonic; n = 2 gives the second harmonic or first overtone, and so on.