A ha-ha is a recessed landscape design element which creates a vertical barrier whilst preserving views. The design includes a turfed incline which slopes downward to a sharply vertical face, typically a masonry retaining wall. Ha-ha's are used in landscape design to prevent access to a garden, for example by grazing livestock, without obstructing views.
The name "ha-ha" derives from the unexpected/ amusing moment of discovery when, on approach, the recessed wall suddenly becomes visible. (Not so amusing, of course, if you didn't see it and fell over it - I personally would then be saying something other than "ha-ha".)
You can just see a couple of sheep in the field beyond the ha-ha.
Are you somewhere warm by any chance?