Work through the 4 stages in order. Start with the easy case of adding a positive number, and build up one step at a time to the trickiest case, "subtracting a negative." Once you've got it down, mix things up freely in Free Exploration.
Ever wondered about the negative numbers used for below-freezing temperatures, or basement parking levels? Integers make a lot more sense on a number line, with positives lined up to the right of 0 and negatives to the left. Adding is like moving right, and subtracting is like moving left.
But when you're adding or subtracting a negative number, the direction flips once more. Adding a negative actually moves you left, and subtracting a negative actually moves you right. It's the same idea as canceling a "go down" button in an elevator — you end up heading back up instead.
Work through Stage 1 to Stage 4 in order, watching which direction the arrow moves and by how much. Once you've got the hang of it, change the numbers freely in Free Exploration and check that the rule always holds.