The SVD is a small hobbyist hardware device that emulates a floppy drive and floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

that is suitable for attachment to an old computer such as a TRS-80 or Apple 2.
Floppy disk images are downloaded to the SVD through a PC

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

's serial port (RS232).
The SVD presents these images as physical floppies to the old computer, allowing it to be booted or to otherwise run
old software.
Used mostly by retrocomputing

Retrocomputing

Retrocomputing is the use of early computer hardware and software today. Retrocomputing is usually classed as a hobby and recreation rather than a practical application of technology; enthusiasts often collect rare and valuable hardware and software for sentimental reasons...

 enthusiasts, the SVD provides a mechanism
by which archived software can be reliably and easily loaded on the old computer equipment, aiding
in the process of restoring and using it. Unlike using floppy images with emulator

Emulator

In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

s running
on a PC, the SVD is used with physical hardware.