Within the MMO, Super Adventure Box is described as a training simulation designed to educate young members of the industrious, diminutive Asuran race. Outside of Guild Wars 2 canon however, Super Adventure Box is best known as a joke that found unexpected success. Earlier this year, developer ArenaNet created a commercial for the then-fictional platformer as an April Fools' Day joke. The commercial was an instant hit with fans, so as part of the developer's commitment to launching new content for Guild Wars 2 every two weeks, ArenaNet developed Super Adventure Box in earnest, first releasing it as a browser-based 2D platformer, then as a full-fledged 3D addition to Guild Wars 2.
For more information on Super Adventure Box and the rewards it offers, have a look at ArenaNet's handy study guide.