NP-completeness is a concept in computer science that helps classify problems based on their difficulty. A problem is considered NP-complete if it is both in the class NP (nondeterministic polynomial time) and as hard as the hardest problems in NP. This means that if you can find a quick solution for one NP-complete problem, you can find quick solutions for all problems in NP. To determine if a problem is NP-complete, researchers often use a technique called reduction. This involves taking a known NP-complete problem and transforming it into the new problem in a way that preserves the solution. If this transformation is possible, the new problem is also NP-complete, indicating its complexity.