Hess's Law states that the enthalpy change for a reaction that occurs in many steps is the same as if it occurred in one step.
Another way to put this is if several reactions add up to some total reaction, then their enthalpy changes will add up to the enthalpy change for the total reaction.
∆Htotal = ∆Hrxn 1 + ∆Hrxn 2 + ∆Hrxn 3 + etc.
Hess's law problems usually give you two or three reactions with their enthalpy change information, then ask you to find the enthalpy change for some target reaction.
You must figure out how to make the given reactions add up to the target. This can mean reversing the reactions (and reversing the sign on the enthalpy change), or using them multiple times, or both.