It really just goes to what containers are. You can think of them as at a simple level as tars of chroots you can pass around and that an ecosystem of tooling has built up around.

This allows you to easily deploy generic ones you download from the community, and since they usually ship the script that was used to build it, it's easy to extend them with your own changes by creating your own script that uses a generic public one (the base, not the script) as a starting point, allowing everyone to share work. (e.g. get an nginx container and do the small bits to customize it for you in a script, and you don't need to worry about the building or system setup of container)

In addition to the normal chroot stuff, it uses cgroups functionality on linux to constrain it even more such as limiting CPU and RAM or disk IO, and can lock down a bunch of functionality from even root inside, and you also usually have to specifically allow whatever internal ports are listening externally (and not necessarily on the same port).

So containers are what you get when you take the idea of a chroot and try to extend and build upon it towards the direction of a true virtual machine, but without going to the step of fully emulating hardware. The benefit is that they are super lightweight and can more easily share CPU and RAM. The downside is that you're exposing more of your base system kernel to it, so it's likely somewhat less secure than a true hypervisor (but very likely more secure than just running those processes on the base system).

So, now that we have a shared base of what a container is (sort of, I'm not sure enough on the specifics to assume I didn't make some missteps in that description), rootless containers extend that so that a regular system user can run a container which internally runs as root, but not the real parent system root, but it works for the container because the kernel uses namespaces to allow it. This is both cool because it allows for root to be used even less on a system and scary because as another comment in this thread noted, it exposes some kernel APIs to "root" users (run and controlled by system users) which might not normally be checked as much for security problems since you have to be root already to use them.