What is the 'iddat', ‘un-Islamic’ marriage, that led to conviction of Imran Khan and wife Bushra Bibi?

The recent judgment, delivered on a Saturday, pertains to the "un-Islamic marriage case," initiated by the ex-husband of Bushra Bibi. This marks the third conviction for Khan within a week, all while he is detained in Rawalpindi prison since August, facing over a hundred charges.