Say's law proposes that supply creates its own demand. This means that the income derived from producing certain goods by some, allows them to purchase goods produced by others. Since all people have a need to purchase goods, they will seek to produce some goods to derive income and buy whatever they want. Thus the product markets will always necessarily be in equilibrium.
Workers who earn income, earn that income in order to be able to buy a variety of products they want. Thus, by working and producing goods, these workers generate the income with which these goods can be purchased.