It's the audio track in a channel. A channel is the horizontal strip, a waveform box is a piece of audio somewhere across that strip. You can double click on one to enter single editor mode, or single click to make it active (once it's active you can drag it around, apply effects to it, move it on a different channel etc etc).

Sorry still working on improving the UX :)

EDIT: There's a short video here - https://x.com/pkalogiros/status/2053492761350046032