An abstraction describes what something should do without actually showing you how to do it. According to the Microsoft documentation:
Included as examples of abstractions are abstract classes and interfaces. Examples of abstractions in the .NET Framework include Stream, IEnumerable<T>, and Object. If the topic of abstraction seems a bit fuzzy now, don't worry. I will go into much more detail in the section on encapsulation and the difference between encapsulation and abstraction.