Allows you to use non-lsp tools like shellcheck, python's black, clang-format and so on with the nvim lsp interface.
https://github.com/jose-elias-alvarez/null-ls.nvim/blob/main/doc/BUILTINS.md
A huge list of tools have "built-in" support. You simply install them and add the name to your nvim config. I haven't had to do so, but it seems you can create your own configs for commands that aren't builtin.