Think of it like buying a pre-built computer versus building your own computer. When you buy a pre-built computer, it comes with everything already decided for you. You bring it home, plug it in, and turn it on and all the software has already been installed. On the other hand, when you build your own system you get to choose each piece of hardware, put it together, select your operating system, and install your own software. Building your own system takes more work, and you cannot simply turn it on and begin using it. But, in exchange for this additional effort, you get self-determination and control over your hardware and software. You can choose your favorite operating system and the apps you want to install, no longer beholden to the manufacturer’s one-size-fits-all selections. Similarly, with a Self-Managed VPS, you have to do some initial set-up and later management. Yet, you get to choose the software configuration you want to perfectly suit your operating specifications.
A VPS runs its own copy of an operating system (OS), and customers may have superuser-level access to that operating system instance, so they can install almost any software that runs on that OS. For many purposes they are functionally equivalent to a dedicated physical server, and being software-defined, are able to be much more easily created and configured. They are priced much lower than an equivalent physical server. However, as they share the underlying physical hardware with other VPSes, performance may be lower, depending on the workload of any other executing virtual machines.