A root sum is to continuously sum up the digits of a combo until you are left with a single digit.
For example if 965 hit you would get 9+6+5 = 20. Then you do 2+0 = 2. You now have a single digit so the combo 965 has a root sum of 2.
You can break the 1000 combos in Pick 3 into the following root sums
Root Sum 0: has 1 combo 000
Root Sum 1: has 111 combos
Root Sum 2: has 111 combos
Root Sum 3: has 111 combos
Root Sum 4: has 111 combos
Root Sum 5: has 111 combos
Root Sum 6: has 111 combos
Root Sum 7: has 111 combos
Root Sum 8: has 111 combos
Root Sum 9: has 111 combos
A Root Sum Set contains more than 1 Root Sum.
In my latest snapshot Root Sum Set {3, 6, 7} in Kansas has skipped 19 consecutive draws. Each root sum (except 0) represents 111 combos. So in this example Root Sum Set {3,6,7} represents 333 combos (111 x 3), and none of those 333 combos has hit in 19 draws. Once the skip count approaches 20 you should considers playing combos from those 333 combos since they are way overdue to hit.
Its just a way to do a quick analysis of what root sums are missing beyond just a single root sum but multiple of them at a time.
Another example is roulette you can think of the 3 columns on the betting layout as representing a third of the numbers with one of the columns not hitting in 19 spins of the wheel. You might be inclined to start placing bets on those numbers in the column that have not hit since its beginning to be a statistical outlier.
That is about the best I can explain it.