Complex sentence: The Fourier transform decomposes a function of time (a "signal") into the frequencies that make it up, similarly to how a musical chord can be expressed as the amplitude (or loudness) of its constituent notes.
Split into simpler sentences: The Fourier transform decomposes a function of time (a "signal") into the frequencies that make it up. This is similar to the way in which a musical chord can be expressed as the amplitude (or loudness) of its constituent notes.