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Target Disk Mode is a boot mode unique to Macintosh computers.
When a Mac that supports Target Disk Mode is started with the 'T' key held down, its operating system does not boot. Instead, the Mac's firmware enables its drives to behave as a SCSI, FireWire, Thunderbolt and/or USB-C external mass storage device.
A Mac booted in Target Mode can be attached to the port of any other computer - Mac or PC - where it will appear as an external device. Hard drives within the target Mac, for example, can be formatted, partitioned, etc., exactly like any other external drive. Some computers will also make their internal CD/DVD drives and other internal and external peripheral hardware available to the host computer.
Target Disk Mode is useful for accessing the contents of a Mac which cannot load its own operating system. Target Disk Mode is the preferred form of old-computer to new-computer interconnect used by Apple's Migration Assistant. Migration Assistant supports Ethernet or Wi-Fi, which TDM does not. Neither supports USB; however, Thunderbolt-to-Firewire, Thunderbolt-to-Gigabit-Ethernet, and USB-3.0-to-Gigabit-Ethernet adapters are an option when one of the computers does not have Firewire or Thunderbolt.