FDS stands for Famicom Disk System.
It's a special Famicom-like (Famicom is the Japanese name of Nintendo Entertainment System) console where games are loaded from diskettes.

FDS images are different from NES images in that they are usually named .fds, not .nes, and that you also need a disk system BIOS image (usually named as disksys.rom for emulators).

This site does not distribute FDS images or disksys.rom.

A special ROM named disksys.rom is required to play FDS files (files ending in .fds). It should be about 8K in size. If it is not named disksys.rom, it is the wrong file. Do not rename any files. Place disksys.rom in your FCEU folder. Then run the FDS file in FCEUX.

Do not use so-called "FDS hacks". They do not work.