This is an absolutely core concept for the game of tennis. You need to master the splitstep so it becomes instinct – when you are playing you cannot be consciously thinking about your feet, but it is essential for you to not be flat footed, and to be even a moderately quick player.
The split step is a little hop you do right as your opponent is about to hit the ball. You bring your feet both just slightly off the ground at the same time. You land back onto the ground at the exact time your opponent hits the ball. As you land, your legs will naturally coil like a spring.
As soon as your opponent hits the ball, you should be able to tell within a microsecond whether the ball will be coming to your forehand or backhand. As soon as you know which direction it is going, you turn your shoulders and begin to move towards the ball.