The side pass (referred to as a leg yield when applied at walk, trot or canter) is a very important movement to master as it allows you to yield your horse left or right at any moment, allowing for more suppleness through their body and more rideability.
To learn how to position your body to ask the side pass, check out my "Correct Position for Sidepass" video, in my CORRECT POSITION FOR LATERAL MOVEMENTS video series.