Home Manager is a Nix tool for managing configuration for a single user, usually on a Linux or MacOS system, or possibly WSL. You configure installed programs, program configuration (such as dot files), and a number of other things, and you get a reproducible environment that's easy to apply to multiple machines, or to roll back configuration, etc. I find it helpful for having a clear record of how everything is set up. It's the sort of thing that people sometimes use GNU Stow or Ansible for, but it's much more powerful.

A Home Manager configuration is very similar to a NixOS configuration, except that NixOS configures the entire system instead of just configuring user level stuff. (The lines do blur in Nix because unlike traditional package managers where packages are installed at the system level, using Nix packages can be installed at the system, user, project, or shell session level.) Home Manager is often paired with NixOS. Or on Macs Home Manager is often paired with nix-darwin. As I mentioned, the Home Manager portion of my config is portable to OSes other than NixOS. In my case I'm sharing it in another Linux distro, but you can also use Home Manager to share configurations between Linux, MacOS, and WSL.