An idea is a certain way of organizing thoughts that makes the brain happy* for whatever reason.

People are very good at coming up with ideas - given any observation, people can make up any number of ways to explain it. This helps people to figure out small problems, but it unfortunately also creates a huge amount of totally wrong beliefs - myth, superstition, etc.

Many mental disorders seem connected to ideas - schizophrenic people have too many ideas, depressed people have only bad ideas.

* by "happy" I mean a brain state that people will attempt to maintain.