Bare metal is a term used to refer to a computer that has no software installed on it. It consists only of its hardware components, and perhaps firmware such as a BIOS.
The distinction often comes up in the context of restoring a backup or resetting a machine. If a system can boot from its hard disk, then many options for backup restoration may be possible, ranging from restoring individual files using the backup software running in the operating system, to reinstalling the operating system using the information stored in a recovery partition.
On a “bare metal” system, there is nothing on the hard disk, and no other software on the machine in order to perform restore or recovery operations. Repair or recovery then usually requires a rescue or restore CD or USB device from which to boot. A machine whose hard drive has failed and been replaced with an empty one is usually in a “bare metal” state.
A machine sold without a pre-installed operating system can also be referred to as a “bare metal” machine.