Cognitive Complexity is a measure used by Sonar to help developers understand how complex and hard-to-read their code has become over time. It's calculated based on the number of loops, branching statements, and nesting within the code, which increase complexity as they are added. The model takes into account that some constructs, like useful design patterns, do not increase complexity and can be ignored in the scoring. Sonar provides a way to visualize this complexity through its tools, such as SonarLint or SonarQube, allowing developers to identify areas of their code that need improvement. By understanding Cognitive Complexity, developers can write more maintainable and easier-to-understand code, making it better for both themselves and their team in the future.