At its most basic level, is a "keyboard smash." These are the six keys located in the center of the home row on a standard QWERTY keyboard.
The term is much more clinical and has a defined meaning in the world of software and gaming. It refers to a version of a program (usually a large video game) that has been highly compressed. Fghjkl Meaning REPACK
If you are looking at a file list on a torrent site or a direct download forum and see "Fghjkl [REPACK]," it could simply be a corrupted file name or an uploader being humorous. It signals that the content is a compressed installer, but the "fghjkl" serves as a "null" or "unspecified" identifier for the software inside. At its most basic level, is a "keyboard smash
Are you trying to something, or are you just curious about the slang? Did it appear alongside a specific game or software title? If you are looking at a file list