Simple Definition:

The Rule of Law is a principle that states that everyone is subject to the law and that the law should be applied fairly and consistently [in a regular and predictable manner].

For example, if someone commits a crime, they will be arrested and go through a fair trial in court. Likewise, if someone follows the law, they will be protected and treated equally under the law.

Very Simple Definition:

The Rule of Law means that everyone has to follow the law and the law should be applied fairly.

For example, if someone does something wrong, they will get in trouble. And if someone does something right, they will be safe.