File I/O (Input/Output) refers to the ability to read data from files and write data to files on a computer's storage. This is essential any time your program needs to save information that persists beyond the current session — like saving a player's progress, logging events, storing settings, or loading level data.

Reading a file brings its contents into your program's memory so you can work with it. Writing to a file saves data from your program out to storage. Most languages also support appending — adding new content to the end of an existing file without overwriting what's already there.

File I/O is one of the areas where languages differ quite noticeably in both syntax and approach. Some handle it in just a couple of lines, others require more setup. It's also an area where error handling becomes especially important — files might not exist, paths might be wrong, or you might not have permission to access them — so you'll often see File I/O paired with try/catch blocks.