Peasants grew the crops and tended the livestock. They usually rented land from lords of manors. They pay their rent to these lords in the form of either livestock or harvest. They made money by selling the surplus they produce, so that they can buy goods that they need. Eventually, if they make enough surplus and sell them, they can pay off the Lord with cash, instead of just livestock and harvest, which is more convenient both for the lord and the peasant.
Sometimes, poorer peasants who cannot afford to rent land work for wealthier peasants, so that they can help they can help the wealthier peasants pay the lord, make money and also make money themselves without paying a lord.