Are there some guidelines of what people should be using the chat for, like discussions strictly related to Bitcoins, or is it more of a free chat with people from this SE, so some off-topic is allowed?
Chat doesn't have a strictly-regulated purpose, per se. It's a "third place".
The third place is a term used in the concept of community building to refer to social surroundings separate from the two usual social environments of home and the workplace. In his influential book The Great Good Place, Ray Oldenburg argues that third places are important for civil society, democracy, civic engagement, and establishing feelings of a sense of place.
Chat is the coffee shop you hang out at with colleages after a long day of work at the Q&A factory. Sometimes you bring work from the office into the more casual social surrounding, but sometimes you want to talk about other things of interest to those you work with.
It's and informal public gathering place. A space for people who love the topic to meet, discuss, and collaborate in a different way than you typically see on the main site. It's there to complement everything from the strict Q&A and meta discussion, to the desire just to hang out with the people you see on the boards everyday.