The os module is a built-in Python library that provides a portable way to use operating system-dependent functionality. It allows programmers to interact with the underlying operating system that Python is running on, enabling tasks such as creating and removing directories, fetching directory contents, changing the current working directory, and constructing file paths robustly. In the context of data preprocessing, the os module is essential for preparing the file system, such as creating destination folders for datasets before writing data files to them.