In Western tonal music, primary chords are defined as one of the three triads built on the first, fourth and fifth tones of the scale. These are the chords that define the key of a composition because they are the typically the most prominent chords.
Western tonal music, whether popular or classical, operates within a certain key. All the notes within that key have a relationship with the key, as do the triads built upon that note. For example, in the key of C, the first triad is a C major triad. It is called the tonic triad. The chord built on the fifth tone, G, is G major. It is called the dominant triad. The chord built on the fourth tone, F major, is called the sub-dominant triad. Using only the natural notes of the scale, these are the only tones that result in major triads. All others produce minor or diminished triads. This is true in minor keys as well as major keys. The relationship between these three major triads is fundamental to tonal music. The progression known as I, IV, V, I (tonic, sub-dominant, dominant, tonic) is probably the most familiar chord progression in all of Western music.