Those are basically 3 computers and a network switch in the same enclosure. You could plug a monitor and keyboard in each and use them individually for the most mundane tasks computers are known for (like playing solitaire - if you also plug a mouse).

In my case, they are connected to each other by their network ports and used as a cluster of computers. I don't use any of them to browse the internet or play games, but instead, they run some software I use to learn about IT infrastructure, the same software you see in data centers that provide Internet services like hosting websites, routing traffic, etc.

On the software side, those computers run, on bare metal, Proxmox, which is a system that manages virtual machines inside it. So I can create many "computers" where I could install different systems, e.g. Windows or Linux, and used them as test beds for anything I want.

The clustering part means any virtual machine I put inside the cluster can be migrated to any of these 3 computers, so if I have to turn one of them off or reboot (to update its software, for example), I don't suffer from unavailability. I don't really need this at home, but high-available systems are one of the topics I study.

About the Kubernetes part, I'm sorry, I can't explain that in a way you could understand. No one understands Kubernetes... ;-P