Each is a timed 5km run which takes place at 9:00 on Saturday morning (except for Glasgow, who evidently get up half an hour later). They're timed well to get yourself started over the weekend, without getting in the way of other events, you get a mix of abilities which means that you'll usually have someone around you which helps you keep going, and since the course is the same you can measure your improvement as time progresses. Or lack thereof - I feel I've been generally backing away from my goal of completion in under 20 minutes rather than advancing toward it.
If you're close to one, and quite like running, I'd encourage you to give it a go. They're fun.