What is CPU Throttle?

We enabled the Observability Plus add-on today and now we can see a graph labeled “CPU Throttle” with durations on Y-axis and timestamps on the X-axis. What is actual definition of CPU Throttle used by this chart? (This is under Observability → Serverless Functions → Advanced metrics.) I can’t seem to find it anywhere. It’s definitely not “were your functions throttled and returning 429s” because that under Usage shows zero data.