In video games, a save file is a computer file (or series of files) that contain all the information needed for the player to resume playing the game. As video games have evolved over the years, save files have gone from merely representing checkpoints (so that you'd restart at the beginning of a level), to being able to resume in the immediate situation that you saved in (very useful for RPGs).
Because they exist outside the tightly-controlled confines of a programmed environment, save files are at the mercy of the storage device they are saved to. Because of the nature of these storage devices, corruption can occur, where the save file has been damaged to an extent that the game is unable to parse the data correctly, preventing the save file from being loaded.
However, intentional corruption is possible by precision-editing specific locations within a file(s), often for the purposes of cheating. This was easier in the earlier days of video gaming on PCs, as save files were often written in plain text. However, to prevent this form of cheating, many games now scramble the save files to prevent this being done, although this is somewhat of an arms race; as programmers get better at obfuscating save files, other programmers get better at de-scrambling them (especially the case for AAA titles, such as the Mass Effect series).