A dynamic turn is when your board gets out away from underneath the body. If you were to draw a line tracing the path of the board's edge through the snow and compare that to a line tracing the path of your belly button over the snow, they would be very different lines. For basic turns, the body never gets more than a foot away from over the top of the board and the 2 lines would have a similar shape.
It is common for dynamic turns to have sequential edge change and leg bend/extend movements instead of simultaneous movements. So instead of changing from toe to hell edge with both feet at the same time, the front foot changes first then the back foot. As the front of the board passes underneath the body, the front leg is bent and the board is flat on the snow. But the back leg is still long, sticking out away from the body and still on edge. As the front foot goes away from the body and the front leg extends, the front of the board starts going onto the new edge. When the back leg passes underneath the body, the back leg bends and the back of the board goes flat to the snow. As the back leg goes out away from the body, the front leg is coming back. If you see someone doing this, the common observation is that they are making "snaky turns".
Most beginning riders make "basic" turns by moving both feet from edge to edge and bending or extending both legs at the same time. It's possible to make dynamic turns on flatter terrain, but it is easier to learn them on slopes with more pitch. When you make dynamic short raidus turns, you don't do any steering with your feet. By controlling the edge angle of the board, getting very high edge angles and bending the board you get all of the directional change that you need.