Ok so I know we had a threat about the concept, but here is the definition in plain words:

People use the word “freelance” quite a lot these days, generally to refer to someone who doesn’t have a “real” job. But that’s hardly an accurate definition.

The word freelance comes from the Middle Ages, when there were basically two types of knights. There were the knights who worked exclusively for one king. Then there were the “free lancers,” or knights who worked for anyone who would pay them.

The idea of freelancing is still with us, but kings have been replaced by businesses, while knights have been replaced by professionals of all kinds. Today there are more freelancers than ever before and more freelancing opportunities as well. But it’s important to have a firm understanding of what freelance really means today.

Here’s a good definition:

A freelancer or freelance worker is a person who pursues a profession without a long-term commitment to any one employer.

This describes freelancing pretty well. The part about having no long-term commitment to one employer is easy to understand. It is quite different from being a full-time employee.
Read more here:
What does "freelance" really mean?