It shrinks the original bulky .bin/.cue or .iso files into a much smaller size without losing any game data.

: The compressed ROM format. CHD (Compressed Hunks of Data) is the gold standard for disc-based retro emulation. It shrinks the original PlayStation .bin / .cue files by up to 50% without losing any data or audio quality, saving massive amounts of storage space on your SD card. The ACME Toolkit: Genius Puzzle Mechanics

: Point your emulator directory to your .chd file.