a type 1 hypervisor
A type 1 hypervisor runs directly on the bare hardware, with no ordinary operating system underneath it — it is the first and lowest piece of software the machine loads after firmware. Picture an apartment building that was designed from the ground up to be apartments: there is no single original house that was later subdivided; the building manager sits at the foundation and every tenant is a guest from day one. That is a type 1 (bare-metal) hypervisor: the host has no separate OS of its own; the hypervisor is the host.
Because it sits right on the hardware, a type 1 hypervisor talks to the CPU, memory, disks, and network cards itself, and schedules the virtual machines on the real CPU with very little in the way. It keeps a thin, tightly controlled code base whose only job is to host VMs, which means less overhead and a smaller attack surface than running a full OS plus a hosted hypervisor. Real examples are Xen, VMware ESXi, and Microsoft Hyper-V; these are what power most public clouds and corporate data centres, where many VMs must run fast and stay isolated on one box.
The trade-off is that a type 1 hypervisor needs proper hardware support and drivers for the machine it runs on, and it is not something you casually install on a laptop just to try one other OS — for that you would use a type 2 hosted hypervisor instead. The payoff is performance and density: because there is no extra host operating system competing for resources, type 1 hypervisors get the most virtual machines running well on the least hardware.
A cloud company installs VMware ESXi straight onto a brand-new rack server — there is no Windows or Linux installed for people to log into. ESXi is the only operating-system-level software on the metal, and its entire purpose is to boot and run dozens of customer virtual machines.
No host OS underneath — the bare-metal hypervisor is the lowest software on the machine.
Real type 1 hypervisors are not always perfectly thin: some (like Xen) run a special privileged helper VM, often a Linux, to host device drivers. So the boundary between type 1 and type 2 is a useful classification, not an airtight law.