It's like a conference, but there isn't an attendance fee or pre-scheduled presenters. At the beginning, there's a big board with a grid of timeslots and rooms. If you want to give a talk, you write down your name and what the talk is about on a sheet of paper and stick it in one of the grid squares. If it sounds interesting, people will show up.

BarCamp started as sort of a reaction to an O'Reiley event called FooCamp which was invitation-only, and in Portland it ran for a number of years until it got replaced by Open Source Bridge which ran for a few more years. I'm not aware of anything like it in Portland now, which is a shame.

https://en.wikipedia.org/wiki/Foo_Camp