Real GDP shows the value of an economy’s output, adjusted for any changes in inflation, interest rates or other factors which could affect price levels. By tracking the demand for goods and services, aggregate demand can help to show what quantity of goods and services will be purchased at various price levels.
As a result, aggregate demand is also an indicator of whether spending in a given economy is high or low. High spending could mean that people are not currently incentivised to save, perhaps because interest rates are low, or inflation is expected to rise. Low spending could mean that people are currently saving more, perhaps because interest rates are high, or inflation is under control.
Aggregate demand curve explained
The aggregate demand curve represents the total quantity of goods and services which are currently in demand at different price levels. It is usually assumed that the curve will slope downward because of the law of demand, which states that the demand for a good will decrease alongside an increase in price.
You could draw a line from the price axis to the aggregate demand curve and see how the price level corresponds to the real GDP. This is demonstrated in the below graphic.