According to Wikipedia, a ha-ha is a ‘recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond from the other side. The design can include a turfed incline that slopes downward to a sharply vertical face’.

Or as author Terry Pratchett put it slightly more bluntly: ha-has are ditches to prevent inconvenient peasants from accidentally wandering over and spoiling the view for wealthy landowners.