Zangief shifts his body back during a jump, then he drops his knees down, hitting his opponent.
This is one of Zangief's two crossup attacks, alongside Flying Body Attack. This attack comes off a light rather than a fierce, so it has less block and hitstun compared to his other crossup. What makes this worth using is that it actually shifts Zangief's position backwards slightly in his jump. While "FBA" will also connect in this situation the reason why you would want to use "DKD" is that since it moves his position slightly it will actually change what direction the opponent has to block (As in, some ranges Knee Drop will hit left and Body Attack will hit right). This is one of the reasons why Zangief has such a powerful knockdown game.
Another benefit is that it actually retracts his hurtbox slightly. This makes him capable in some situations of jumping forward, using knee drop to avoid an anti air, and then use one of his Command Grabs immediately upon landing.