There are two major ways in which having domesticated animals helps farmers produce more crops.
First, having domesticated animals gives the farmers fertilizer. The dung from the animals can be used to fertilize the crops. When this is done, the crops can grow better and more food can be produced.
Second, in the days before machines, animals were really important for being able to farm some types of land. Animals can pull plows and that allows farmers to break up and plant soil that could not be planted if farmers were just using their own muscle power (and things like digging sticks) to break up the soil.