A monopoly is a situation where a single company or entity has complete control over a particular market [a specific area or industry where goods or services are bought and sold], with no competition.
For example, if there was only one company that sold smartphones and no other companies were allowed to enter the market [start selling smartphones], that company would have a monopoly on smartphone sales.
Another example is a company that is the only provider [company that supplies something, such as electricity or internet connection] of electricity in a town, with no other options for consumers.
Very Simple Definition:
A monopoly is when one company has complete control over something and no one else can do the same thing.
For example, if there was only one place to buy ice cream and no other place was allowed to sell it, that place would have a monopoly on ice cream sales.