Glossary
Menu bar
The system strip at the top of a Mac screen, where status items live.
The menu bar holds the Apple menu, the front app's menus, and status items on the right: clock, control center, and third-party readouts. A menu bar app is one whose ongoing status lives there, sometimes with a fuller window behind it.
Disk Housekeeper puts free space in the menu bar so you can see it without opening Storage. The figure on the app page has no sample number, because the number is your disk's. Low-space alerts are the companion when that readout gets tight.