A file extension is the suffix at the end of a filename (e.g., .txt, .jpg, .pdf) which typically indicates the file's format and tells macOS which application should open it. Changing a file extension modifies this suffix. macOS generally hides common extensions by default in Finder for a cleaner look, but revealing them is necessary before changing. Altering an extension doesn't convert the file's content; it simply tells macOS how to try and interpret it.

For example, you might change an image file named picture.jpeg to picture.jpg to match a system expecting that specific extension. Similarly, a text document named notes.rtf could be renamed notes.txt to open it in simpler text editors instead of a rich text application. This is commonly done in web development, graphic design, or when preparing data files for specific software requirements.

The main advantage is compatibility with specific software or processes expecting a particular extension. However, a significant limitation is that changing only the extension doesn't alter the actual file data; inappropriate changes can render files unopenable. Exercise caution to avoid changing critical system file extensions. Ensure you know the correct target format and consider that making extensions visible permanently provides more control over file management.