If you honestly do not know what the absolute value of a number is, which is what you appear to be saying, you have no hope of doing this problem. Talk to your teacher about it!
If you do know the absolute value function then you know that |x- y|= x- y as long as [itex]x\ge y[/itex] and |x-y|= y- x if x< y. The region [itex]0\le x\le 1[/itex], [itex]0\le y\le 1[/itex] is a square. [itex]x\ge y[/itex] below the diagonal from (0, 0) to (1, 1) and y> x above the diagonal. Integrate those two separately and add.