Glossary
Low-space alert
A notice that free space on the Mac is low, separate from actually cleaning.
An alert is a signal, not an action. Disk Housekeeper can raise one when free space is low, next to the menu bar readout that shows the number. This site does not publish the threshold, because the product description does not state a gigabyte cutoff.
The alert does not silently run Smart Clean or Autopilot. You still preview a clean, and removals still go to the Trash by default. If you came here from a full disk, start with Free up space on a MacBook.