The basic principle of supply and demand can be looked at as a scale.
When people don't want a product (low demand) but there is lots of it (high supply) the price is cheap or non-existent. An example of low demand and high supply is dandelions, they're medicinal and good tasting but they grow like a weed so demand is low. This is why dandelion medicine is cheap/free.
When people do want a product (high demand) and there is not a lot of it (low supply) the price is expensive. An example of this is Ferrari, there are only 8,400 made each year, for a planet with almost 8 billion people! This low supply makes people feel that they are getting something exclusive that will make them stand out from the crowd so they are willing to pay more for it, increasing demand. This is why Ferrari's are expensive.
Most products fall somewhere between the two extremes I've listed.
Resource Videos (click on the words below):
What is something in your house that is in high demand and low supply?
What is something in your house that is in low demand and high supply?