My humble take on all this:
He is talking about an Aggregate Supply and Demand curve.
When a supply curve is horizontal that means that an increase in demand, meaning moving the the demand curve to the right will not change the price of anything, because the intersection of the new demand curve with the supply curve is still at the same height, or price
The question is, why are we justified drawing the supply curve as horizontal in the first place? What kind of strange reality is that? After all, won’t a given price induce exactly one quantity of suppliers? How can we say that for a whole range of possible values of supply, the price will stay the same? Won’t competition reduce the price as supply increases?
Your prof’s explanation is that we have high unemployment. Uncle Wikipedia explains. First, why the supply curve stays horizontal as you move to the right from a given initial point:
The Keynesian aggregate supply curve shows that the AS curve is significantly horizontal implying that the firm will supply whatever amount of goods is demanded at a particular price level during an economic depression. The idea behind that is because there is unemployment, firms can readily obtain as much labour as they want at that current wage and production can increase without any additional costs (e.g. machines are idle which can simply be turned on). Firms’ average costs of production therefore are assumed not to change as their output level changes.
In other words, he is assuming that costs of production are the only determinant of price, from the point of view of the supplier. This is true very often, because due to competition, suppliers have to keep down their price to costs of production plus some standard mark up, because that’s what the competition is doing.
In other words, prices are rock bottom already, and will stay the same until costs of production go up. And costs go up mainly because of higher wages, which will go up only when there is full employment. Until then people will accept the going wage rather than be unemployed.
Wiki continues:
This provides a rationale for Keynesians’ support for government intervention. The total output of an economy can decline without the price level declining.
The Austrian take on all this? That there is a little boo-boo in that picture, mainly that it assumes costs of production will stay constant as supply goes down. But that isn’t so. If supply is less, then, at the same price, less money is coming in for everyone, the supplier as well as the provider of his resources. This will put pressure on costs of production to go down.
Keynesians are no fools. They know this, and their rejoinder is that the biggest cost of production is wages. And wages don’t go down, ever. Wikipedia mentions this:
This provides a rationale for Keynesians’ support for government intervention. The total output of an economy can decline without the price level declining; this fact, in conjunction with the Keynesian belief of wages being inflexible downwards, clarifies the need for government stimulus.
The Austrian rejoinder? Yes, if wages are inflexible downward, and won’t go down when they have to, we are in a pickle and will have chronic unemployment. For sure. But what makes wages inflexible downward? One thing. Govt intervention. Long story why.