These overlays (sometimes known as bezels) display art on top of the game you're playing. To use them, you need a PC or Raspberry Pi (or other compatible system) running some version of MAME and the associated game software, as well as a widescreen (16x9) monitor mounted vertically. To display the overlay while playing a game, download the artwork file for that game overlay (it must have the same filename as the game), place it in the Artwork folder within your MAME directory and launch the game. (For instance, the software for the game Magic Sword is called msword.zip, so it requires a file in the Artwork directory also called msword.zip.)
Some theme packs include versions for RetroArch. If you are using RetroArch, find the RetroArch folder in the pack download, or click the RETRO buttons on individual pack pages. Watch this video to learn how to install RetroArch overlays.
Most of these overlays include multiple presets that let you choose the size of the game screen, access a "dark" version designed to look more like a dark arcade, and sometimes other options. To choose a preset, press Tab while running the game to bring up the MAME menu, select Video Options, and select a preset from the list.