3. Basic FFT (Fast Fourier Transform).
FFT is a mathematical process to decompose any signal into a series of sinusoidal signals.

Instead of representing the graphical result of the waves in the time domain, it is represented by traces in the frequency domain. Each trace represents the amplitude of a wave and its frequency.

The decomposition of any signal usually generates an infinite number of sinusoidal signals.

If we add all those sinusoidal waves, we would obtain the initial signal.

If we add the first sinusoidal waves, we would obtain the initial distorted signal.

If the initial signal were formed by a certain number of pure sinusoidal signals, in its representation we will have that certain number finite of traces.

In addition to the graph of Amplitudes in the frequency domain, we can also obtain another graph with the phase shifts of each of the signals, we will not see that in this tutorial.

FFT is used in telecommunications, sounds, music,...
This is very basic information, check out other tutorials on this topic.