If one stops and holds time still, one can observe the world as it is. What one sees at that point is the current stock. For example, at any one point in time, one observes only one set of prices. Businesses list their stocks of cash, accounts receivable, inventory, and fixed capital in their balance sheet statements. The sum of all assets is total wealth.
Stock variables are money supply, food, clothing, bonds, and savings accounts.
As money is exchanged for other assets, and vice versa, income and expenditure adapt passively. A flow is something that occurs over time. One cannot observe a flow at a point in time, but rather only as time passes. Over time, one earns income and one makes expenditures. What one does not spend, one saves.