Abstraction

Abstraction means focusing on what an object does, rather than how it does it.

Example

public abstract class Animal
{
    public abstract void Speak();
}

public class Dog : Animal
{
    public override void Speak() => Console.WriteLine("Woof!");
}

The Animal class defines what every animal should do (Speak),but not how it does it.
Each subclass, like Dog, provides its own implementation.