In a nutshell, a unikernel like MirageOS lets developers of an application build a kind of "micro OS" that is specialized just for their application. So rather than needing the entire Linux or MacOS system just to run a simple application, the unikernel approach just bundles up the pieces of the OS the specific application needs (like networking, file storage, DB, etc) into an extremely minimal, portable thing.

If you are familiar with VMs or Docker containers, that's the right sort of arena to be imagining.

You could imagine a unikernel Nostr relay. It wouldn't need an entire Linux OS to run the simple relay - the relay software would live on top of just the system requirements it needs and that tiny thing would be run "as if it were a whole OS".. if that makes sense?