Online means a heartbeat in the last 90 seconds from a tab
that is actually visible. A backgrounded tab stops counting, so this is people looking at a
page, not tabs left open.
Unique counts a visitor id stored in that browser. The same
person on a phone and a laptop is two; a cleared browser starts again. No IP address, no
user agent and no query strings are stored, and Do Not Track is honoured — so these numbers
are a floor, not a ceiling.
Windows are rolling, not calendar days: "24H" means the
last 24 hours from now.