An arpeggio is a group of notes within a chord which are played one after the other, either going up (ascending) or going down (descending). An arpeggio is sometimes referred to as a broken chord. Executing an arpeggio requires the bass player to play the notes of the chord individually in order to differentiate them. The notes in the arpeggio all belong to that particular chord. The chord may be a simple chord which might consist of the 1st, 3rd, and 5th notes or "degrees" of the scale, or it may extend further into a more complex arpeggio consisting of the 1st, 3rd, 5th, 7th, or even the 9th, 11th or 13th degrees of the scale.
The simple chord is called a triad. Starting on the first note of the C scale (or tonic), and building an arpeggio from that first scale degree, an arpeggio of this three note type of chord, or triad, would be C, E, and G. It is arrived at by skipping every other note of the C major scale. It can also be referred to as 1, 3, and 5 of the C major scale, for the first, third and fifth degrees of the scale. This also holds true for all minor chords and their respective scales.