Mod managers are external tools that make handling RimWorld mods more comfortable. Their core function is to manage and sort your modlist from outside the game without any need to launch every time you change something. That's just the basics though; depending on the tool, you'll get a bunch of extra features that can greatly help with modding and troubleshooting game-issues. As of now, only two public mod managers exist:
RimPy
RimPy was the popular choice for years, but it hasn't been maintained for years now, which caused even the main feature - the auto-sorting of mods - to degrade. RimWorld's 1.6 update additionally broke other popular features in addition. For the average user it unfortunately isn't recommended anymore.
RimSort
RimSort is RimPy's open-source successor and the current recommendation for most players. It's community-maintained and while not flawless provides what matters: reliable sorting & proper maintenance.
If all you're using are 15-20 mods, you might want to consider not using any mod manager at all - but for everyone planning on using larger modlists, those tools are simply too convenient as to miss out on them. There are also a few private mod managers floating around in the community (usually made by modders for personal use.) They're typically for more experienced users though, so we won't dive deep into them here. This guide is currently also extremely barebones and is planned to be expanded later - including examples of their most useful features (like here!) and how to use them.