Short answer
HubSpot's KB: the membership log shows records added to or removed from a segment over time, with the segment version for each change. In a static segment, add or remove records directly from the log.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Pair it with version comparison when a filter change is the likely cause.
3. Related
Active vs static segments and views to segments.