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....
Solus
The Solus is a 2 VCO, monophonic analog synthesizer with a 3-octave, 37-note
(C-C) keyboard. It, like most ARP synths, features mostly sliders instead of
knobs. It was built into a flight-case, with all of its inputs and outputs on the inside
of the case lid.
Both VCO's feature a slider selecting variable waveforms from sawtooth to pulse,
as well as a slider controlling pulse-width and the amount of pulse-width
modulation. VCO2 is syncable to VCO1 and can be detuned. The pulse-width
can be modulated by the LFO or the ADSR. The VCF (four-pole, lowpass filter)
has controls for frequency, resonance, as well as three sliders controlling
modulation amount from the keyboard, the LFO, and the ADSR. It also features a
digital ring-modulator and a pitch-bend knob. The back panel features interfaces
(in and out) for CV, Gate, and Trig. Also in the back is a footpedal input and
External Audio input, as well as high and low outputs.