Zero is a six-sided, or hexagonal, if you want to sound fancy, bright blue polyester string. It is lubricated for more string movement, which is supposed to increase spin potential. It comes in two different gauges, 1.23 and 1.28. Go thicker for durability and control and thinner for more power and comfort. I would say it is a medium to soft poly.
https://tennisnerd.net/gear/strings/string-reviews/restring-zero-review/31681