Decompose any signal into its frequency components
Any periodic signal can be represented as a sum of sine waves at different frequencies. The Fourier Transform reveals these hidden frequency components.
Time Domain (left): Shows the signal as it varies over time - what you'd see on an oscilloscope.
Frequency Domain (right): Shows the amplitude of each frequency component - what you'd see on a spectrum analyzer.
Try building a square wave by adding odd harmonics (1st, 3rd, 5th...) with decreasing amplitudes!