Over time, one earns income and one makes expenditures. What one does not spend, one saves. Saving is loaned to households, who borrow to buy real estate and consumer goods, to businesses, who borrow to buy capital goods, to governments, who borrow to buy social overhead capital and pay military and civilian personnel, and to foreigners, who borrow for the same reasons. One cannot see income, spending, saving, investment, borrowing, or lending, except as time passes. Flow variables are found on income statements.
Flow variables are income, spending, and saving.
As income is spent, the stock of assets adapt passively. While the two terms have separate and distinct time interpretations, they are linked in the following sense. The next day is a holiday. He does not work, he earns no income. However, to survive, the person consumes a can of beans and four hot-dogs on rolls.