Slide on your side is a swimming drill to get you started swimming freestyle more on the sides while also maintaining a good balance and a long body.
This drill belongs to an older sequence of swimming drills to learn the freestyle stroke. Please consult the Learn How To Swim The Freestyle Stroke article to learn the current sequence of swimming drills.
Learning how to swim the freestyle stroke more on the sides is a great way to reduce drag and become faster. However swimming on the sides feels unnatural at first. But with this drill you will quickly become comfortable sliding up and down the pool in this position. Have fun!
If done correctly, this drill allows you to slide easily in the water with a relaxed kick. This is because this position unites the three key swimming skills to reduce drag:
You may notice that it's easier to keep balance and slide on one side than on the other. This is often the case and perfectly normal. For example, personally I have better balance on the right side.
The causes for this can be multiple: one lung is smaller than the other, one side of your body is stronger than the other, etc. Just drill some more on the weaker side to compensate if you find yourself in this situation.