What is it?
FHS is short for Filesystem Hierarchy Standard. Essentially, it just describes where a program should look for shared libraries, etc.
I use NixOS now, and it isn’t FHS compliant. This can cause issues when trying to run non-Nix packaged software. I hit a wall with this, while trying to run MATLAB and some SW for my Master’s thesis. The software will simply not run and I don’t want to deal with workarounds.