Cachix manages the prebuilt binaries used by IHP. If you don't install Cachix, nix will compile a lot of IHP's dependencies from scratch. This takes a long while and is not recommended.
Both of those things are scary and dangerous. I don't know what your software or Cachix is doing.
Can you tell me exactly what it will be doing to my system? What changes? Does it use a lot of disk space? Is it easy to undo? And is there anyone who recommends doing it who isn't associated to IHP?