Hi gregeet,
Tencel is a semi-synthetic viscose fabric made from the pulp of Eucalyptus trees and is a trade name for Lyocell made by Lenzing. It it a very smooth fiber with a soft feel and breathes and wicks moisture very well so it would be a good choice for temperature regulation (similar to cotton and much better than synthetic fibers such as polyester). It is also more environmentally friendly than many other viscose/rayon types of fibers.
There is more about the different types of fibers and fabrics in post #7 here which is about sheets and bedding but would apply to fabrics used in mattress covers/ticking as well.
It is certainly a high quality fabric that is very popular because of how it feels, its strength, and like other cellulosic fibers that are either natural (such as cotton or linen) or man made from cellulosic sources (such as bamboo rayon or micromodal made from beech trees) it’s ability to wick moisture and help regulate temperature.
As you know I think highly of Quality Sleep Shop and you certainly made a high quality/value choice … and congratulations on your new mattress 
Phoenix