It's as simple as ABC.
Imagine there is a 100 bread and 1000 people. 1000 people want bread but there is only 100 bread. In this case demand is high but supply is low. It's easy to identify because when demand is high and supply is low, price is increasing. The bread seller will sell these breads to people who pay the most from these 1000 people. To sum up, only those 100 people from 1000 people who have the highest budget, will be able to buy 100 bread.
Now imagine that there is a 1000 bread and 100 people. In this case demand is low but supply is high. There is more bread than more people who are going to buy bread. Since bread seller won't be able to sell 1000 bread easily to 100 people, he will try to make bread as cheap as possible to sell as much as possible.

This is how supply and demand work. When price increases, demand increases and when price decreases, demand decreases.