Location is a point. Radius turns this point into an area. When player enters this area, task activates (opens).
By default radius for tasks is 10 meters. The bigger the task radius is, the easier it is to activate it, but the farther away from actual location it will activate. The smaller the radius is, the more difficult it is to activate it (because GPS is inherently inaccurate), but the risk of activating the task too far away is reduced.
In some cases you want to have really big areas for activation (when using locations on the road). If you need to guide players very close to an object use the intro (pre-clue) for that. Locations with radius of 5 meters or less are very hard to activate. Note that if you set radius to 0 meters, then it will not activate by GPS, but still activates on iBeacons or QR-codes.