It needs warmth, moisture and food to grow.
Yeast ferments sugars into alcohol and carbon dioxides gas.
There is two types yeast commonly available nowadays. Regular active dry yeast and instant yeast . Both have been dried to deactivation, but the yeast cells are not killed. Active dry yeast has a hard outer shell that needs to be softened in warm water with some sweetener for 5-10 minutes before adding to other ingredients. There are fans of both types of yeast.