concept

A concept is a general idea or mental representation of something. Our concepts are based on our experiences and on what we know about examples of that concept.

For example:

It is hard for children to understand a complex concept such as 'democracy', but they can understand simple concepts like 'good' and 'bad' which are relevant to children's lives.

Abstract concepts such as 'truth' or 'beauty' can be hard to understand. Also, different people may understand the same concepts differently.