Alice and Bob both start at the same number ("10") and both do half of a sum, which means they both get the same result without seeing what the other person added (15 and 30). This is useful in cryptography because Alice and Bob do not share their private numbers, which means a third party cannot spy on the result (55) unless they can find both private numbers; even if a third party knows Alice sent "10" + 15 = 25, they don't know the result is 55 unless they also know Bob sent 30.