They mean being able to group several windows together as tabs, rather than tabs implemented in specific applications.
Just like a bunch of windows stacked on top of each other that move and resize together with tabs in the title bar to switch between them
Here’s Haiku’s explanation (this also covers the tile feature which I had forgotten about but is also super useful): https://www.haiku-os.org/docs/userguide/en/gui.html#stack-tile