A ha-ha, or haha (supposedly named for the reaction
people had on seeing one), is essentially a large ditch built
in place of a fence, to give the appearance that the garden and
surrounding lands are as one. It seems that they were introduced into
the UK from France in the 18th century by Lancelot
'Capability' Brown, or possibly earlier by Charles
Bridgeman. They were part of a movement in gardening away from
formal gardens to a more 'natural' style of landscaping.
As King says, 'there's an actual ha-ha (in the novel), of course, and it plays a major role in the story, but the symbolic relevance is the presence of a huge unaddressed fissuretraumatic brain injuryin the landscape of the protagonist's life.'
When asked where the idea for his first novel came from he replied, 'My brother Hank was autistic; he was six years older than me....