What does the phrase undoing the operations in reverse of the order of operations mean?

This might refer to the method used to solve an equation. Take, for example, the equation:

2x + 1 = 15

To solve this for x, the "x" must be isolated on the left. The multiplication and addition must be "undone" - that is, the inverse operation must be applied. Also, since multiplication is carried out before addition, this "undoing" should be applied in the inverse order - in this case, undo the addition first. What you end up doing, in this case, is first, subtract one from both sides, leaving you with:

2x = 14

and then, divide both sides by 2.