Counterpoint discusses the methods used over the centuries, particularly in the Renaissance and Baroque periods, to compose well-balanced polyphonic music. As opposed to the newer theories of Harmony that analyze music as vertical chords and are best adapted to homophony, Counterpoint portrays composition as an interweaving of two or more independent voices, with rules and suggestions on which intervals and types of movement between the various voices provide the most pleasing results. 

It is highly recommended to work your way through this page starting at the top, as each installment builds on the previous one. These videos are just an introduction. If you really get interested in this topic, I suggest signing up for an actual class in Counterpoint at a nearby music school or university.