The split step is very important for your footwork in tennis as it allows you to get to the ball quicker. The split stance is a ready position which means your feet are about shoulder distance apart or even a little wider and your knees are slightly bent so you are lower in height to which you normally stand.
The split step allows your body to get in an explosive position to take off quickly as soon as you know which direction the ball is coming to you. .
For your split step you simply do a little jump off the ground just before your opponent makes contact so you can land at the earliest time. You know which direction the ball is coming to you and this allows your legs to be in coiled which lets your legs have stored energy to quickly explode to the ball.