The phrase "teaspoon yeast" refers to a small amount of a microorganism called yeast that is used in baking to help dough rise. A teaspoon is a measurement unit that denotes a small quantity. Full definition
In bowl of stand mixer fitted with whisk attachment, whisk together water, honey, whole - wheat flour, and 1/2 teaspoon yeast until very smooth, about 3 minutes. (epicurious.com)
(I am a pastry chef so I made my own) 12 ounces warm water 2 teaspoons salt 1 - 2 teaspoons yeast if you like. (kingarthurflour.com)
if you want a bit more rise, next time you make the dough, add another 1/4 to 1/2 teaspoon yeast rather than the long rise time.) (ruhlman.com)