挨个儿 is an adverb meaning 'one by one' or 'in turn', used to describe doing something to each item or person in a sequence. It is the erhua (儿化) variant of 挨个 (āi gè), and the final 'r' is pronounced as a retroflex suffix attached to 'gè'. You can use it in both formal and informal contexts, often before a verb to indicate the order of action.