A channel manager keeps a property's availability calendar in sync across every platform it's listed on — Airbnb, Booking.com, Vrbo, a direct-booking site — so a booking confirmed on one platform blocks those same dates everywhere else, close to immediately.
The problem it solves
Without one, a listing can stay live and bookable on a second platform during a window that's already taken on the first. Most platforms offer an iCal export/import as a lighter-weight alternative, but iCal feeds typically refresh only periodically (hourly, sometimes less often), which leaves a real window where a new booking on one side hasn't yet blocked the other. A proper channel manager treats every platform as one shared calendar rather than several kept loosely in sync.
Standalone tool vs. a feature of something bigger
"Channel manager" can mean a dedicated standalone product, or it can mean a feature built into broader property management software. For a host running one to a handful of properties, calendar sync as a built-in feature of the tool they're already using for bookings and expenses is usually simpler than adding a separate, dedicated channel-management product on top.