Yeasts are unicellular fungi that can multiply by budding. Many yeasts are important to us: baker's yeasts, for example, help various baked goods to raise and make beer for us. To do this they convert various sugars into alcohol and carbon dioxide.
Explanation for children:
Yeasts are fungi that are very important for the production of food. Without yeast there would be no fluffy bread or pizza, for example, because they help doughs rise.