A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
The time-domain and frequency-domain plots of a sine wave
A single-frequency sine wave is not useful in data communication. We need to send a composite signal, a signal composed of many simple sine waves.
According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes and phases.
A composite periodic signal
Decomposition of a composite periodic signal in the time and frequency domain