Thunderstorms often develop updrafts strong enough to overcome the force of gravity long enough to send raindrops up into the colder tops of the clouds where they freeze. Eventually the hail stones become heavy enough that gravity overcomes the force of the updraft and they fall to the ground. Small hail is a relatively common occurrence during summer.
Very large hail, however, is very rare, at least in any one location. But almost every summer, hailstones the size of baseballs or even softballs fall somewhere in the Dakotas or Minnesota. Hail this big requires an updraft of around 100 mph or higher. Such an updraft is difficult to maintain and so these super-large hail stones are rare in any one location.
Fortunately, only a very few of us will see one in our lifetimes.