The CPU line is showing CPU frametimes measured in milliseconds, not in percentages. In other words, the line is showing how long the CPU takes to prepare a frame for the GPU to render.
You’ll notice in my videos it takes the CPU 16.6 milliseconds to prepare a frame. This is due to me limiting the frame rate to 60fps.
1000ms = 1 second
1000ms ÷ 60fps = 16.6ms
The GPU is fed instructions by the CPU. If the CPU stalls, this will cause the GPU to also stall. Which will result in stutter and lower frame rates.
This is why CPU spikes are important. A CPU spike in the yellow graph represents the CPU taking too long to prepare a frame for the GPU. So instead of the CPU taking 16.6ms to prepare a frame, it might spike to 18ms or more. Which is too long for maintaining a FPS target of 60fps (16.6ms).
So if the yellow line is flat, that means the CPU is delivering frames to the GPU in perfectly even spaced intervals. Which translates into no stutter and butter smooth gamplay. So long as the GPU can also render it’s frames in 16.6ms also.