In this lesson, Hank Smith shows you how to add harmony to your melodies to achieve a richer, fatter sound. Harmony a combination of two or more notes to form a chord. A good way to think about it is a choir. The different sections of a choir sing different notes which blend together. On your banjo, each string can represent a different section of the choir.
Hank uses the familiar tune "My Country Tis Of Thee" to demonstrate. He guides you through playing the melody. Then adds another note on a different string, eventually having you play 4 notes at the same time.
Let us know if this lesson helps you and if you have any questions or things you would like demonstrated in the comments below. Good luck!