In most cases the worst that happens is you’d be exposing things you share on your network, so they’d have access to any files or folders that are shared among the other computers on your network under certain circumstances.
I don’t believe anyone would have access to the information you’ve looked at online, unless it was through some sort of logging. As in they could see you went to fluther, and possibly information you’ve posted there, but anything that was encrypted would still be encrypted. And that’s only if they were looking at the web traffic directly which would take more than access to your wifi network. It would be the first step though.
Most commonly the worry is that someone will use your network to do things they wouldn’t want to do on their own. Whether that’s downloading huge files, or something more damaging. Usually there isn’t much of a threat to you. It’s just letting someone on your network can be the first step, so people generally lock that down as a first level of defense.
That’s not to say other things couldn’t be exposed, but it’s not due to the wifi network itself.