macOS's kernel-level API that tells apps which directories changed, and when
FSEvents is the macOS API that notifies applications about changes to the file system, streaming a coalesced log of which directories were modified and in what order.
The kernel maintains a per-volume event log at /.fseventsd/ and exposes it through the FSEventStream APIs in CoreServices. Applications register interest in one or more paths and receive a callback whenever files underneath them are created, modified, renamed, or removed. The API is directory-granular by default — you learn that something in ~/Documents/Projects/EditA changed, then you scan that directory to find what. It is also persistent: an app that was quit or asleep can replay events from a stored event ID when it wakes back up, so nothing is missed across reboots.
For any continuous-backup product, FSEvents is the difference between watching your disk efficiently and hammering it. Without it, the only way to know what changed is to walk the entire file tree and stat every file — on a 4 TB working drive with a million Lightroom previews that is minutes of I/O per pass. With FSEvents, the backup engine sits idle until the kernel tells it something happened, then scans only the affected folders.
In macup, the backup engine subscribes to FSEvents for every path in your backup set and persists the last processed event ID across restarts. When you drop a new RAW into your active shoot folder, the engine is notified within seconds, scans that one folder, and chunks just what changed — not your whole library.