The Juke Step is essentially just strafing quickly in random directions to "dodge bullets".
When: While firing/Being fired at.
What: A series of quick moves to the right, and left.
Why: To throw off an attackers aim, while minimizing the effect on your aim. Your under fire, and out in the open, with no cover in sight. You juke step to throw off their aim while returning fire.
Note: The meaning of this move is to take yourself out of the sight of the enemy, giving you time to sight him, and disable him. Firing back is essential.
How: Using your strafe keys quickly tap them, alternating each tap. The result will be you quickly sidestepping fire.