转告 means 'to pass on a message' by word of mouth — you received it and then tell it to someone else. It is the everyday term for relaying a verbal message, while 传达 (chuán dá) is more formal (often official orders) and 转达 (zhuǎn dá) is common for passing on greetings or regards. Unlike the broad 传 (chuán), 转告 is specifically about spoken messages.