Abstract nouns are names of things we cannot grasp (recognize) with any of our five senses. We can grasp such things only with our mind.
We use our senses to recognize human beings, animals, material things, i.e. things which occupy space. We call such things concrete. The names of such things are called concrete nouns in grammar. (You already know that nouns are names.)
Unlike these concrete things, we have concepts like justice, honesty, presidency, ministry, childhood, happiness, intelligence etc. It is our mind which grasps (i.e. understands) these concepts.
We cannot say, that these concepts are not real. They are immaterial, not in the sense that they don't matter, but in the sense that they aren't about space-occupying matter. Politicians are willing to take great risks to get hold of the Presidency or the Prime Ministership.
To make it easy to understand, people often substitute a concrete thing for these abstract concepts. They might say, that the politicians are fighting for 'the chair'. But we know, that they are not referring to a chair which we can buy from a furniture shop!
Some of us enjoyed our childhood; others sadly did not...childhood was real and we have happy or sad memories about it. But can you see or touch or taste your childhood?
When we talk about such immaterial things, we need labels (i.e. names). The names of such concepts or ideas are what we call abstract nouns in grammar.
Why Do We Call Them "Abstract"?
It is because we arrive at such concepts through the mental process of abstraction.
To understand abstraction if you are keen...
Before I explain abstraction, I must deal with the difference in meaning between essential and important.
For something to be itself, there are essential parts of it and inessential.
By inessential I don't mean unimportant. Some things are important but not essential. The heart and the brain are essential for human life, a hand is not. We can live without a hand even though it is very important.
So then, what is abstraction?
It is a mental process by which we remove the inessential, one by one, and retain what is essential for a thing to be what it is. We thus come to an idea or concept, which is a collection of essential (necessary) things for something to be itself. The name given to the resulting idea or concept is the Abstract Noun.
For example, we have seen many children (children are concrete) and we have through the process of abstraction come to the concept of childhood. All that is essential to be a child is childhood. There can be no child without childhood. Childhood is an abstract concept and the name childhood is the abstract noun.
