"Air strafing, often referred to as simply strafing, is one of the core movement mechanics in Counter Strike. Air strafing allows you to fully control your movement mid-air while gaining speed in the process."
To airstrafe, synchronize your mouse movement with your keyboard movement. For example, to strafe left, hold the A key on your keyboard and move your mouse towards the left. To strafe right, hold the D key and move your mouse towards the right. It is vital to let go of the W key while strafing. If done properly, you should turn while midair to your strafe direction while gaining some speed. Also, it is wise not to strafe while ducked as your max turn speed decreases when holding duck.
Your keyboard strafe key should be held down for the entire duration of the strafe. However, how far you turn while strafing is controlled by your mouse movements. The more you move your mouse in one direction, while holding the proper strafe key, the more you will turn.
Strafing is the foundation of another important KZ element: long jumps. Since strafing increases your speed while midair, that equates to an overall farther jump distance. However, a single strafe takes you to another direction that is often unintended in a longjump. This is remedied by strafing back and forth in opposite directions to counteract the unintended direction, while gaining even more speed with each strafe.