There's an old saying that goes, "One is the loneliest number." Non-programmers usually start counting at one, making it the first number in the count.

Programmers generally have to start counting at 0. So to them, the first number is 0. So the joke is that a non-programmer would say "one is the loneliest number" a programmer would say "zero is the loneliest number" because the numbers they start with when counting are different.