What is an Abstraction?
An abstraction is a way to think of a group of elements by referring instead to a set of common properties that they share.
For example, both squares and circles are geometric shapes, thus we say a "geometric shape" is an abstraction of what squares and circles are. Similarly, a "power plant" is an abstraction of a concrete type of power plant, such as coal power plant or a nuclear power plant. You can't tell someone to just build "a power plant" because a power plant, being an abstract concept, doesn't concretely exist in the real world. Whatever power plant they build will end up being a specific type of power plant and not the abstraction itself.
Which abstraction we want to use—what properties we want to abstract—depends entirely on what we'll use that abstraction for. As abstractions can be very vague in human languages, it's always a good idea to explicit define what you are abstracting and for what purpose.