An arpeggio consists of the chord tones for a chord played individually one after the other. This way of playing a chord is called arpeggiating the chord. On the other hand, if all the notes in a chord are played at the same time, then the chord is said to be a block chord. You can think of arpeggios as chords that have been "melted down" to their individual chord tones, and block chords as "frozen" arpeggios.
Sometimes the melody of a song is accompanied by a guitar or piano playing arpeggiated chords. In addition, arpeggios are often found within the melodies themselves, especially in folk songs, anthems and religious music. Very often, the tones of the melody are also found in the chord playing along with the melody, particularly on the strong beats.
By training your ear to identify arpeggio patterns when you hear them in melodies, it will give you a good clue as to what chord accompanies that part of the melody.
Phrase Fitter will train you to recognize arpeggios when you hear them in a song, and to quickly identify the chord which matches each arpeggio.