mypy is an optional static type checker for the Python programming language. It helps developers identify type errors in their code before runtime, making it easier to catch bugs early in the development process. By adding type annotations to their code, programmers can provide mypy with information about the expected data types, which enhances code readability and maintainability. Using mypy can improve collaboration among team members, as it clarifies the intended use of variables and functions. It integrates seamlessly with existing Python codebases, allowing developers to gradually adopt type checking without needing to rewrite their entire code.