Many synths can only play one note at a time (they are monophonic). With the arpeggiator on, if more than one note is pressed at a time, the synth will alternate between the notes. They also often have a range control that allows the user to tell the ynth to play additional octaves - so if the range is set to two octaves the synth will play the keys that are held down in the octave that they are played and then in the next octave up/down....
AKS
This is one of the classic pieces in analogue synth history. Brian Eno still caries one
around and Klaus Schule Schulze still uses his on every recording. The pin-block
patching is a little strange at first, but the versatility is like that of a modular unit. You
can patch anything into anything and the built-in reverb, while a little cheezy, can be
anywhere in the signal chain.