A filter filters out certain frequencies in the sound. There are four main types: a low-pass, high-pass, band-pass and notch, which are covered elseware in the glossary. Filters have two controls - the frequency control selects at which frequency the filter should start operating and the resonance control creates a peak just before the bit where it starts filtering out frequencies. This is used to create acid-type sounds....
Axxe
The Axxe is basically a monophonic version of the Odyssey (which was
duophonic). It came with "Lessons in Electronic Music" (a 155 page textbook),
"Instant Axxe" sheets that you could lay over the front panel that showed you
patches, "Axxe blanks" (blank patch diagrams), an interface diagram (to hook it
up with other synths), and a book of 50 patches.
The ARP Axxe (like the Odyssey) is also equipped with "Proportional Pitch
Control (PPC), ARP's latest contribution to more expressive synthesizer
performance. The triple-pad, pressure-sensitive controller lets you bend notes
sharp and flat, and add vibrato, all without moving a single slider or switch."