CHD files are a type of compressed file that was originally developed for use with MAME (Multiple Arcade Machine Emulator). They are designed to store large amounts of data, such as game ROMs, in a compressed format that is easier to manage and transfer.
If you own original PS1 discs and want to convert them to CHD to use in other emulators (or to store until ePSXe finally updates), follow this guide. epsxe chd files
Here is the critical answer to the keyword query. CHD files are a type of compressed file
How to Play Playstation 1 Games on PC - Duckstation Full Guide such as game ROMs