Abstraction is the process of separating ideas from instances of those ideas. Or something generic (abstract) from something concrete.
An example is a blueprint being an abstraction of a house. A house is a real, physical, existing object. You cannot perform the same actions on a blueprint of a house as you could on the actual house. (e.g. you could open a door on a house, but you cannot open the door on a house that is on a blueprint)
That idea and concept of abstraction is something unique to humans. Humans can classify dogs, cats, elephants, hamsters, etc. as mammals, as they all have traits that we associate with mammals. But to a different species such as a dog, these are all very different things and the dog will treat them as such. You could not teach the dog that they are all mammals.
The idea of inheritance will classify a platypus and a rodent as mammals. There is no such thing as a “mammal”, but there are types of mammals, likewise, there is no such thing as a “beaver”, but there are more specific types of beavers, such as an American Beaver. So mammal and beaver are no more than abstract thoughts or concepts. Animal is even more abstract than mammal, rodent, or beaver. The idea of classifying and organizing is one of creating abstractions in a hierarchy such as this.
A vehicle is another example of abstraction, branching from the polymorphism document, you could go to town using a car, a bicycle, or by simply walking. These three things are different, but will all get you to your destination. The abstract idea in this case is the mode of transportation, as your feet, a car, and a bicycle are all modes of transportation that will do the same thing
One final example of abstraction would be a photograph. If someone owns a photograph of themself, that isn’t really them - but rather an abstraction of them.