I don’t understand aggregate demand

RUSS ROBERTS doesn't understand aggregate demand:

Does an increase in aggregate demand increase employment?

Yes, if by an increase in aggregate demand you mean people buying and selling more from each other where buying and selling includes consumers and manufacturers.

The above statement has virtually no informational content. It is equivalent to saying that when the economy is healthy, there is lots of exchange going on. When an economy is not healthy, there is less exchange. There is less buying and selling of goods and services and labor. To describe that unhealthiness as less aggregate demand is just to put the problem into different words.

You should read the rest of his post.

I don't understand either.