NP-hardness is a classification in computational theory that describes problems for which no efficient solution algorithm is known. Specifically, if a problem is NP-hard, it means that solving it quickly (in polynomial time) would also allow us to solve all problems in the class NP quickly. This makes NP-hard problems particularly challenging, as they are at least as hard as the hardest problems in NP. An example of an NP-hard problem is the Traveling Salesman Problem, where the goal is to find the shortest possible route that visits a set of cities and returns to the origin city. While there are methods to find approximate solutions, no known algorithm can solve all instances of this problem efficiently.