This is an idiom that Jesus uses to mean "yes" without being completely clear about what he means. Alternate translation: "You are saying it" or "You are admitting it" (See: [[rc://en/ta/man/jit/figs-idiom]])
Possible meanings are 1) the phrase "from now on" is an idiom that means they will see the Son of Man in his power at some time in the future or 2) the phrase "from now on" means that from the time of Jesus's trial and onward, Jesus is showing himself to be the Messiah who is powerful and victorious.
Here "Power" is metonym that represents God. To sit at the "right hand of God" is a symbolic action of receiving great honor and authority from God. Alternate translation: "sitting in the place of honor beside the all-powerful God" (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/translate-symaction]])