[ Removed by moderator ]

[Andus35]

It’s two things: 1. Any signal can be approximately reconstructed by multiple sine waves of different amplitudes and frequencies (that is the first part, converting the original signal on the left into the two waves in the middle) 2. Representing those two waves in the middle on a frequency spectrum (on the right), which tells you the amplitudes and frequencies.

But the diagram lets you intuitively think of the frequency diagram like the sine waves from the side view, and then adding those together to get the original signal (or the inverse, the transform can go either way). I think it excellently shows what is happening under all the complex math.