Arp-X8 is an octal arpeggiator - that is, eight arpeggiators all rolled into one. An arpeggiator is a device that scans a musical keyboard and then plays the pressed notes in sequence, thus forming an electronic approximation of the traditional arpeggio.
While most arpeggiators offer limited control over the arpeggiation - usually an up/down button and maybe a range control - Arp-X8 expands on that considerably by offering 10 different motion algorithms, along with rhythm controls and a host of other variations. Each of the eight arpeggiators can have entirely different settings, and can even play on independent MIDI channels, for incredibly dense clouds of sound.
Arp-X8 is also designed to work closely with Seq-303 - also on the CD - and other members of the Techno Toys family, allowing you to create entire compositions in real time.