A
signal can be viewed from two different standpoints; the time domain or the frequency domain.
The one which we are most used to is the time domain. This is like the trace on an oscilloscope where the vertical deflection is the signals amplitude, and the horizontal deflection is the time variable.
The second representation is the frequency domain. This is like the trace on a spectrum analyser, where the horizontal deflection is the frequency variable and the vertical deflection is the signals amplitude at that frequency.
Any given signal can be fully described in either of these domains, and we can convert between the two by using the Fourier Transform.