The sum of the individual digits of a number, repeating this process until we get a one-digit result.

Example: 3456

Sum the digits: 3 + 4 + 5 + 6 = 18

But 18 has two digits, not one! So we repeat the process: 1 + 8 = 9

Answer: the Digital Root of 3456 is 9

Note: we always end up with a number between 0 and 9.