The University of Massachusetts explains that peristome teeth serve as a valve that allows the spores to exit the capsule. The peristome teeth open and splay outward during times of low humidity, allowing the spores to float away and disperse.
The University of Massachusetts states that the peristome teeth are one of the most characteristic features of mosses. They are formed from the remnants of the cell wall, and are fiber-like in structure. As their name, which literally means "around mouth," implies, peristome teeth form a rim around the mouth of the capsule, according to Wikipedia.
The University of Hamburg explains that all mosses except graphite mosses possess peristome teeth. The teeth are constructed from two different layers. One layer readily absorbs and releases water, while the other layer does not react very much to varying humidity levels. This causes the teeth to bend outward during times of low humidity.
The University of Hamburg states that some mosses have peristome teeth that work in other ways. For example, Funaria mosses have slits between the teeth that expand during dry periods. In other mosses, such as Polystrichum mosses, the peristome teeth do not respond to moisture. Mosses without peristome teeth have evolved other methods for dispersing their spores.