It basically refers to the act of the new emperor or empress demonstrating that he or she has succeeded the predecessor by holding the Sokui no rei (ceremony of the enthronement), and there was no distinction between sokui and senso (accession to the throne) in the past.
基本的には即位の礼を行い、天下に自分が天皇の位を継いだことを示すことで、昔は践祚との区別はなかった。