Higher thread count means a smoother silkier and more luxurious fabric, if everything else is the same (weave, ply, mill, and type of cotton).
You would be excused for thinking that 100s = 100 threads per inch, but that’s incorrect (and a common misconception in the industry). Rather, these numbers refer to the yarn size. 120s means there are 120 hanks (1 hank = 840 yards) of yarn in one pound.
Thread count is often referred to with a number like 50s, 80s, 100s, 120s, 140s 160s, etc up to 200s.