The vertical axis shows the performance index (whether recording marks are well written) of the recording properties called “jitter.” Specifically, it shows the variation of the gap between the recording mark edge and the data clock edge. A smaller value represents fewer errors when reading data.
The horizontal axis shows the number of overwrite cycles. Ordinarily, jitter rapidly worsens after a certain number of overwrite cycles due to recording layer deterioration.

This is why Sony developed a durable recording layer. Jitter hardly rises even after repeated overwrites, resulting in stable recording/playback.