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....
CS-05
The CS-05 is a single oscillator monophonic synthesizer with a 3 octave, 37-note (C-C)
keyboard. the voice architecture consists of a single VCO, a VCF, a LFO, and a simple
envelope generator. The VCO (Voltage Controlled Oscillator) has sliders controlling the level of
it's two waveforms, sawtooth, and pulse. The pulse width can be modulated by the LFO (Low
Frequency Oscillator). The LFO features a sample-and-hold setting and can modulate up into
the audio range for FM sounds. There is an input and a trigger circuit which allows you to trigger
a sound via an external audio signal.