A plant-based sweetener used in some 'natural' toothpastes. It makes the paste palatable without feeding decay bacteria; it has no oral-health effect of its own.
Stevia extracts are intensely sweet compounds from the stevia plant. In toothpaste they replace sweeteners such as saccharin, usually as part of a 'natural' positioning. Like other non-sugar sweeteners, mouth bacteria cannot ferment them into acid.
There is no clinical evidence that stevia in toothpaste improves oral health. Its only role is taste, and toothpastes do not contain sugar anyway.