In one sentence, Barcamp is an ad-hoc gathering of people who love to learn and share, and more than anything they want an open environment where they can perform those freely.
So it is an event where you come to share your opinions and learn from others. Your topic of interest can be anything. Although the first Barcamp was related to more technical and geeky topics, but now over time they have expanded. Now you can talk about anything starting from Olympics to Human Rights, from assembly language programming to Natural Language Processing, anything!