When implementing games, you will often have to work with persistent data—you will need to store the saved game data, load maps, and so on. For that, you have to learn about the mechanisms that let you use the data stored on digital media.
The most basic and low-level mechanism that is used to access data is to save and load it from the files. While you can use the classic file access approaches provided by C and C++, such as stdio or iostream, Qt provides its own wrapper over the file abstraction that hides platform-dependent details and provides a clean API that works across all platforms in a uniform manner.
The two basic classes that you will work with when using files are QDir and QFile. The former represents the contents of a directory, lets you traverse filesystems, creates and remove directories, and finally, access all files in a particular directory.