真的 is a versatile word that works both as an adverb meaning 'really, truly' and as an adjective meaning 'real, genuine'. As an adverb, it modifies verbs or adjectives to emphasize truthfulness (真的吗？'Really?'). As an adjective, it describes nouns (真的东西 'real things'). The POS is listed as adverb, but the adjective use is very common in everyday speech.