First meetup nerves, what should I expect?

There is a casual meetup happening near me next month and I am tempted, but also quite nervous. I have never been to anything like this before and my imagination is running wild in all the unhelpful directions.

What are these actually like in practice? Is it just people chatting over coffee, or is there more to it than that? I suppose I want to know it is relaxed and normal before I commit, and that it is fine to simply turn up, be a bit shy, and see how it goes. Any reassurance from people who have actually been would really help settle me down.