Complexity metrics are quantitative measures used to assess the complexity of software systems or code. They help developers understand how complicated a piece of software is, which can influence maintainability, readability, and the likelihood of bugs. Common metrics include cyclomatic complexity, which counts the number of linearly independent paths through a program, and Halstead metrics, which evaluate the volume and difficulty of the code. These metrics provide insights that can guide software development practices, such as refactoring or simplifying code. By analyzing complexity, teams can make informed decisions to improve software quality and reduce technical debt, ultimately leading to more efficient and reliable systems.