Passacaglia is a form of instrumental music based on the so-called ostinato bass. This, essentially, is a repeated bass pattern. Aside from ostinato bass, one can also transpose the line or vary it with melodic and rhythmic details. However, the basic bass pattern is repeated throughout the composition, and over it, the 'rest' of the music is built. Passacaglia is one instrumental form that is written using such a scheme, and there are also others like chaconne, and the origin of both these kinds of music is unknown, although it is justifiably suspected that they arose as dance compositions wherein a basic repeated bass line would be suited for improvisation of melodies.
Some people may not appreciate the thought of a repetitive bass line, after all, it sounds boring! But nothing is further from the truth. Bass line is just a structure for the musical composition as passacaglia or chaconne is. There are many harmonic and melodic variations built over the bass line that embelish the music. You can hear the possibilities of passacaglia by listening to Bach' s music, where he often could use this essentially free form as a prelude for his fugues, or the interesting Passacaglia and Chaconne by Couperin in his first suite for viols. Usually, a bass line can be introduced on its own or with some simple chords, and as it comes to the repetition, the music is developed further and further. Listen to the examples below to get the feel for it.
This structural framework for passacaglia is just one way to 'keep things together', which is important in isntrumental music. Listening to all these symphonies, rondoes, ritornellos, etc. you hear that somehow the composer maintains the structural integrity of his work. And in passacaglia or chaconne, the bass line is the architectural fundament of the musical art. We need something to follow by our ears, and if there are no spoken words that guide us, there has to be something in music that does the trick. This is how composers like to communicate with us listeners. Of course, you don't need to follow the bass line all the time as you listen to the music, in fact, you could be missing out the rest of the music enjoyment. All the lines in the music are important, but you will be able to recognize the music form by noticing a repeating bass line, and you will know more about the meaning of music you listen to. And that's one more step in the discovering of details of music that makes it more enjoyable to listen to.
Just to complete this discussion, what is the difference between a passacaglia and a chaconne? In the middle of 17th century, it would be about the type of bass line pattern. For example, a chaconne would be based on some this type of bass line, called romanesca bass:
Of course, these basic themes were varied and developed upon, and later on during the baroque era it is hard to really distinguish between a chaconne and a passacaglia.
