The expression $$\frac{1}{1 - mpc}$$ represents the spending multiplier in macroeconomics, which shows how an initial change in spending can lead to a larger overall increase in national income. This formula highlights the relationship between marginal propensity to consume (mpc) and the total effect of fiscal policy on the economy, illustrating that as mpc increases, the multiplier effect also becomes larger. The multiplier effect is a critical concept for understanding how government spending or investment can significantly boost economic activity.