The Appreciation Index or AI is a measure of how much the audience enjoyed the programme. The score, out of a hundred, is compiled by a specially selected panel of around 5,000 people who go online and rate and comment on programmes.
The score of 83 is around average for the Capaldi era. It is equal to The Witch's Familiar, and one point below The Magician's Apprentice and Under the Lake.
[Source: Doctor Who News]