This refers to the Game Boy Light sold only in Japan.

From Wikipedia:

The Game Boy Light was released on April 14, 1998 for ¥6,800[9] and was only available in Japan. Considered the rarest of rare Game Boys, NIB Game Boy Light units regularly sell in the triple-figures on online auction websites. The Game Boy Light is only slightly bigger than the Game Boy Pocket and features an Electroluminescent backlight for low-light conditions. It uses 2 AA batteries, which gave it approximately 20 hours with the light off and 12 with it on. It was available in two standard colors; Gold and Silver, as well as numerous special editions. There is an Astro Boy edition with a clear case with an Astro Boy picture on it. There is a Tezuka Osamu World shop edition with a clear red case and a picture of his characters. There is a Pokémon Center Tokyo version in yellow color.

It was the only handheld Nintendo produced with an integrated backlight until the release of the Nintendo DS in 2004 (The Game Boy Advance SP released in 2003 was frontlit, and the later backlit model Game Boy Advance SP (Model AGS-101) was not released until 2005)