真是 (zhēn shi) is a colloquial verb meaning 'indeed, truly' — it emphasizes the truth of a statement, often with a tone of mild complaint or annoyance (as in 真是的). It is more conversational than 当真 (dàng zhēn 'take seriously') or 着实 (zhuó shí 'truly, severely'), which are more formal and used in writing. 真 (zhēn) is simply an adverb meaning 'really' without the emotional overtone.