Shakespeare's plays is an example of an abstract noun because it refers to an idea not to something tangible as the plays aren't something you can touch. On the other hand, a computer screen is an example of a concrete noun because it is an object you can see and touch.
Concrete nouns are nouns which refer to people, animals or objects, in general to someting specific, such as table, bread, dog, etc.
Abstract nouns are nouns which refer to an idea, such as freedom, sadness, etc.