I am: Compare with study note on 6:49–50. Mark's audience has known from the beginning that Jesus was the Christ, the Son of God (1:1). This was acknowledged:
But this is the first time that Jesus openly and publicly acknowledged that he was the Christ (the Messiah, God's chosen one).
The second part of the reply Jesus gave was that he would sit at God’s right hand, the place of power and honor. This pointed forward to his resurrection and ascension (Luke 24:50–51; Acts 1:9–11; see Philippians 2:9; Hebrews 1:3) and his triumphal coming on the clouds of heaven to judge the world.
At the return of Jesus, roles will be reversed. Those judging the Son of Man will be judged by him.