The e-Reader is an add-on made by Nintendo for its Game Boy Advance portable video game system. It was released in Japan in December 2001, with a North American release following in September 2002.
It has a LED scanner that reads "e-Reader cards", paper cards with specially encoded data printed on them.
Depending on the card and associated game, the e-cards are typically used in a key-like function to unlock secret items, levels, or play mini-games when swiped through the reader. The cards themselves contain data, as opposed to unlocking data already on the device itself.