This is the Psion 3: a handheld PDA from the early 90s. It has room for two flash memory cards. You can run all kinds of programs on these, including a database, word processor, spreadsheet. The Psion 3 Chess Cartridge lets you play this game on the go. It has 2D/3D views and multiple levels of play.
To get started, you simply have to attach the cartridge and install the program. In this video, we test it against Chessnut Air, which happens to be one of the smartest chessboards on the market.