There is a black or gray line that appears across timeline clips that have audio if Settings > Timeline > Show Audio Waveforms is on.
Adjust the volume of a clip by dragging this line up or down. Doing so adds a Gain/Volume audio filter to the clip’s Filters. Double-click it to reset it back to 0 for no change (or you can remove the added filter).
Use Settings > Timeline > Adjust Clip Gain/Volume to turn this feature off.
When the line has not been moved, it represents -1 dBFS. With it you can visualize when the audio level is dangerously close to clipping. However, keep in mind that there can be audio clipping that occurs due to other Gain/Volume filters or mixing with clips on other tracks.
This feature was added in version 25.07.
See also Gain / Volume Audio Filter