Answer: You probably follow a lot of people - and really need this app.
If you are following more than, say, 4,000 people - we need to ask the Twitter API for a lot of information. Rest assured this will all be stored on your device and progressively synchronised each time you open. So please use it a few times, close it and have lunch, and then use again it to unfollow all those accounts because 4,000 is too many!
I was following 3,000 but almost halved it with this app so I don't get that error as much now. I'm still pruning!