A mistrial is basically when a trial gets cancelled, and it's like hitting the reset button. The judge decides that the trial can't continue, and everything starts from scratch. It's not a not guilty verdict, but more like a let's try again kind of thing.
In the case of Lindsay Clancy, the mistrial was declared due to some issues that came up during the trial. We won't bore you with the details, but let's just say it was a bit of a lawyers' nightmare. Now, the case will have to start all over again, which is kind of like Groundhog Day, but without Bill Murray.