Units digit refers to the digit in the one's place of the number. The question is asking which of the number above is the units digit of the difference of the square of a number and the number itself.

Let's have the following test

1^2-1 = 0

2^2-2 = 2

3^2-3 = 6

4^2-4 = 12

It turns out 6 is the units digit of the difference of the square of 3 and 3.