Undercover describes work done in secret, where someone hides who they really are in order to gather information or catch criminals. The classic case is a police officer who pretends to be a drug buyer for months, but the word also covers journalists, government agents and investigators — 'an undercover reporter filmed conditions in the factory'. Two things matter about the meaning. First, it is not just secret; it involves a false identity, actually living as someone else. Second, it is a word about legitimate investigation, so it usually sounds brave or clever rather than dishonest — a spy for a criminal gang would more likely be called a 'mole' or 'plant'. Grammatically it works both as an adjective before a noun ('an undercover agent', 'an undercover operation') and as an adverb after a verb ('she went undercover for six months', 'he worked undercover'). 'Go undercover' is the phrase students should memorise. It can be used lightly and jokingly too — 'I went undercover at my son's school fair' — and that humorous use is common in English. It is not normally used for ordinary secrets: keeping a birthday present hidden is 'secret', not 'undercover'.