Timbre is the tone color of the music. It is what makes a flute sound like a flute and not an oboe. Various sounds that are available in music allow us to hear independent lines with clarity and provide variety within the music. Each timbre creates a unique tone color that allows the listener to identify different elements of the music. A flute may be playing the melody line while the bassoons are playing a separate bass line. Without the ability to recognize timbres, the elements of a composition would be less vibrant and varied. A composer wouldn't be able to create strong powerful brass choirs or lithe and gentle woodwind pastorals.
Sound will manifest itself through vibrations sent through the air. One measure of vibration that determines how loud or soft the pitch will sound is amplitude. When you look at a graph of amplitude you will notice there are wave lines that go above and below the line. One complete cycle will look like a wave that travels above and then below the line before coming back to the center of the line again. The higher amplitudes will have a louder volume, while the lower amplitudes will have a lower volume.
Much like amplitude, frequencies are a cycle of waves that travel along a line graph that measures time. The number of frequency cycles per second directly correlate to how high or low a pitch is. A high frequency will produce a high pitch while a low frequency will produce a lower pitch. The frequency of a pitch is not set in stone and has only become standardized because the US Government standardized Concert A to 440 in 1939. Concert A above middle C as of 2011 has 440 cycles per second. We could also say it has 440 Hz. Hertz is just an easier way of stating that you are referring to the number of cycles. Many orchestras will perform below or slightly above the concert standard of A = 440. What is an A today, may have been a F in 1770.
Melody can be thought of as the horizontal aspect of music. The melody is the part of the music that you could sing if you wanted to. It is essentially a succession of pitch occurring in a consecutive order. Music is a graph, with notes acting as dots along a time line. The vertical aspect of the graph shows how high or low the music is, while the horizontal aspects of music are the amount of time that has passed. When notes are placed along this graph, the musician has a way to identify what pitch to play, how high or low and at what rate of speed.
A gathering of vertical pitches that line up to create chords creates the background texture of the music. If melody is the horizontal aspect of music, then harmony functions as the vertical aspect of music. Most western music has a substantial reliance upon harmony to help set the mood of the piece. In tonal music, there are several types of chords that when played in succession create dramatic pulls in the music like waves on the shore. Dominant chords pull you toward resolutions. While most people do not know the names of individual chords, most everyone can tell when a piece is about to conclude or a particular phrase is ending. This is because of the harmonic pitch elements that are working with the melodic pitch elements to create a natural pull within music.