Five poor summers in a row

[An Inspector Calls]

In the example here, a Fourier transform would translate your signal from the time domain to the frequency domain. If you had a pure sine wave displayed on an oscilloscope you'd be looking at the signal in the time domain - Fourier transform that signal to the frequency domain and you'd get a single spike at the frequency of the sinewave - you're now in the frequency domain. Putting the time domain waveform shown above through a Fourier Transform would bring out all the periody rather well.