Yeast sounds close to the word least. "It's the least he could do" is a common saying meaning it's the smallest, or easiest thing someone could do. Yeast is a type of fungus that is added to bread dough to help make it rise. A baker is someone that bakes bread. The joke is on the word "yeast" and how it sounds like "least". 
A good baker will rise to the occasion , it ' s the yeast he can do .