The supply-and-demand model works well for a single market, one good, one price. Aggregate demand and aggregate supply extend that same basic logic to an entire economy at once: aggregate demand totals everything households, businesses, government and foreign buyers together want to spend across the whole economy at each possible overall price level, and aggregate supply totals everything the economy's producers are collectively willing to produce at each of those same price levels.
Scaling from one market to an entire economy changes what the axes actually mean
In a single-market supply-and-demand diagram, the vertical axis is that one good's price and the horizontal axis is its quantity. In the aggregate model, the vertical axis becomes the economy's overall price level, a summary measure of prices across the whole economy, and the horizontal axis becomes the economy's total real output. That shift changes what actually drives each curve: aggregate demand responds to overall spending power and confidence across the whole economy, not to any single good's own particular substitutes, and aggregate supply responds to the economy's total productive capacity, not to one industry's specific costs.
Where the two curves meet describes the whole economy's equilibrium output and price level
Just as a single market settles where supply and demand cross, the aggregate model treats the whole economy as settling, at least in the short run, where aggregate demand and aggregate supply cross, determining both the economy's overall output and its general price level together. Because this crossing point responds to genuinely economy-wide forces, government spending, consumer confidence, aggregate production costs, that shift entire curves rather than just move along them, this model is exactly the framework economists reach for when analysing recessions, booms and the broad effects of large-scale policy changes on the economy as a whole.
What we're still unsure about
That aggregate demand and aggregate supply extend supply-and-demand reasoning usefully to a whole economy's output and price level is well established, widely used macroeconomic modelling. What's more genuinely a matter of ongoing theoretical debate is exactly how aggregate supply actually behaves in the short run versus the long run, since economists across different schools of macroeconomic thought hold genuinely different views about how quickly wages and prices adjust and how that adjustment speed shapes the aggregate supply curve's actual shape, and this disagreement continues to drive real, substantive differences in how different economists recommend responding to a given economic downturn.
This sits inside Aggregate Demand & Aggregate Supply, one of eight topics in Macroeconomics, one of five domains in Economics, one of seventeen subjects the app can quiz you on.