ELI5:
It lets you remotely control a computer.
It’s different (arguably better) than remote connection software because it is a separate device that basically just forwards your keyboard & mouse inputs. This means that you can control the remote device even if it’s powered off or not able to boot properly, and you can configure the BIOS remotely too.
You could call someone on-site to connect the KVM to a server, but KVMs, while expensive by regular person standards, are pretty cheap as enterprise hardware goes. So some organizations just keep separate KVMs plugged into all critical hardware all the time.
Worth noting here that KVMs are potentially a quite high security risk.
Their website is probably light on details because “KVM” is fairly common industry parlance. If you normally work with this stuff then just hearing those 3 letters tells you most of what you need to know.
Edit: high, not Hugh
Sure, as long as the VPN itself is secure. Strong passwords/keys, etc. A VPN itself can be a potential security risk, as if it’s compromised an attacker can tunnel traffic directly into a network straight past a firewall.
The risks can definitely be mitigated, but if someone’s asking for an ELI5 on KVMs, then it may be best to stay away until they have a better understanding of IT infrastructure altogether.