Harmony simply means a sweet combination of sounds that is agreeable to the human ear. The basic building block of harmony is the chord. To simplify this further, A chord is a combination of three or more notes together that creates tension and/or release to the listener.
The concept of harmony is very broad and depends on the genre of music you are looking at. For this piece, we focus on the popular music category. Chords cannot be examined in isolation, they must be internalized in the context of the scale on which the melody is built. Chords and melody are inseparable twins who work together to create the listening experience for the listener.