File permissions play a critical role in ensuring security and managing access to files in Unix-like operating systems, including Linux and macOS. Understanding file permissions involves recognizing three key categories: the owner of the file, the group associated with the file, and all other users. Each category can have specific permissions defined as read, write, or execute, denoted using indicators or numerical values in octal format. The lesson explains practical commands such as `chmod` for changing permissions and `chown` for altering file ownership, illustrating how to set permissions effectively for sensitive files. Additionally, it highlights the concept of `umask`, which serves as a safety mechanism to limit default permissions on newly created files, emphasizing the importance of proper file permissions to safeguard sensitive data from unauthorized access. Overall, mastering file permissions is essential for any developer working within these systems, as it directly impacts application security and data integrity.