In a simplified economy, income is either spent on the purchase of final goods and services or saved. Expenditure of income on the final goods either causes final consumption expenditure or investment , expenditure.
To the extent income is saved, final goods remain unsold. But it is treated as inventory investment. So, it is said that, aggregate final expenditure of an economy is equal to aggregate factor payments, i.e.
Y= C+S Where, Y = Income
C = Consumption S = Saving