There's some reading you can do on the internet that explains things rather well, like this post on Distrowatch.
What it boils down to is that on an immutable system, the core of the OS or root file system is unchanging/unchangeable by the user. There are quite a few distros like this, and they are becoming more popular too. Fedora Silverblue, Nix, openSUSE microos just to name a few. One thing these all have in common is the heavy utilization of containers or containerized applications.
These systems are great for security, DevOps, and for enterprise Linux. With Nix for example, you can have one config script that will build and load countless identical Nix endpoint machines or servers, making the management of them much simpler for a smaller team. Other areas of enterprise have shifted to container strategies long ago using services like Docker, Podman or LXD.
Having not used one of these distros myself yet, I'm not sure how the average user experience will change. They are exploding in popularity, and it can't all be security and enterprise related, so there's got to be something there. I feel like you'll have all the capability and customization you have with Solus or many other distros now, it will just be at the user level. Apps, icons, themes, all of this will live in userspace, and the base system is essentially segmented from this. It would allow for things like atomic updates too, which could be really helpful.