The Messianic secret is the name given to the peculiar way in which Mark's Gospel contains many references to Jesus trying to keep his actions and his identity secret. Consider how he commands demons and people not to reveal how he has transformed situations at the beginning of the Gospel. You might also like to think about the way in which he gives some teaching privately to his disciples and instructs them not to reveal it to others. Also, interestingly, he tells parables as a deliberate way of stopping others from understanding what he is saying:
With many similar parables Jesus spoke the word to them, as much as they could understand. He did not say anything to them without using a parable. But when he was alone with his own disciples, he explained everything. (4:33-34)
Some critics argue that this reveals the way in which Jesus didn't want to reveal too much too soon, before he himself was aware of the purposes of God through his life. Likewise, using parables offered people time to think about and mull over Jesus's teaching and identity for themselves, rather than telling them directly. If Jesus had been overt from the beginning about his identity, it could have been that he would have had a confrontation with the religious authorities before he was ready for it.