In the real physical world signals are not made in perfect order. This video will demonstrate how Time and Frequency Domains have a direct influence from a signal derived from a laboratory signal generator. The frequency is 263Hz, about middle C on the piano.
The left Time waveform is what you would see in Time Domain and the right Frequency waveform is what you would see in Frequency Domain. The signal is 10 seconds long. Play the video and you can clearly see and hear that the fundamental 263Hz is made up of other harmonics and overtones deviated plus and minus from the center frequency.