Imagine that you live on an island, and the only way anyone on that island can get gasoline to power their cars is through you.
You have established a monopoly on this island, as you are the only one who can sell gasoline and there's no alternative options!
A monopoly is one firm that sells a product with no close substitutes.
In addition, no other people on the island can get access to gasoline. Therefore, you prevent any competitors from entering the landscape!
In monopoly markets, new firms are prevented from entering the industry!