Basically it's a "how to play" screen.
Controls.dat was a project started to document original controls, and control labels (as in how the buttons were labeled on the contorl panel) for arcade games.
Johnny 5 takes info from the controls.dat project, compares it to the input configurations for the emulator in question and displays an image of your control panel (assuming you made a skin) with the buttons/controls/ect used for said game all nice and labeled. It also can display stuff like a real cpo image and such. Xp/2k users can have the info pop-up in game, with the aid of a script.
Emulator developers (not so much you guys, more on the mame end of things) forget that having a perfectly emulated Tron is quite useless unless you have a triggerstick and a spinner, and that emulating a game with 30 buttons, without mentioning anywhere what button does what leads to nothing but a frustrated user that can't play the game.
There are plenty of screenshots over at
http://fe.donkeyfly.com the home of controls.dat, in the viewer forums. 40% of mame's game are supported in the main project, and my plugin has a hand-made file with every game "officially" in daphne with the exception of road blaster, which I can't find enough input info on.