An altered chord is one that contains notes from outside its scale of origin – chords such as b9 or #5 are frequently seen examples of this.
What the example chords are designed to demonstrate is how we might set up a dissonance or ‘tension’, which can then resolve to a more harmonious conclusion. As an interesting side note, film and TV music will often leave a dissonant chord hanging unresolved to underline the tension of what’s happening on screen.
