Complexity theory is a branch of mathematics and computer science that studies how complex systems behave and interact. It helps us understand problems that are difficult to solve, like those found in computer algorithms or biological systems. By analyzing these systems, researchers can identify patterns and predict outcomes, even when the systems are made up of many interconnected parts. In complexity theory, problems are often categorized based on how hard they are to solve. For example, some problems can be solved quickly, while others may take an impractically long time, even with powerful computers. This classification helps scientists and engineers design better algorithms and understand the limits of computation.