即位 is a separable verb (离合词) with two main senses: literally 'take one's seat' (e.g. at a banquet or meeting) and more commonly 'ascend the throne / be enthroned'. The verb 即 and the object 位 can split when duration or aspect markers insert between them — for example 即了位 ('assumed the throne/seat'). You can also insert a modifier like 即皇帝位 ('ascend the emperor's throne'). In modern usage, the 'take a seat' sense is less common than the royal/ceremonial sense.