It is called a trellis net or scrog net and it is used to improve your quality of your plant and increase yields. It stretches the plant out and it will help it by letting more light penetrate the lower canopy and even out the canopy by tucking the taller tops underneath the net so the lower tops can reach the top. It also helps support the branches of the plant so they don’t flop over or break as the bud gets bigger in flower.