The first fugue in the Well-Tempered Clavier is a beautiful introduction to the world of Bach's fugues. A fugue is a style of musical composition where a Theme (melody) is introduced, and then repeated in one or more other voices. This is in contrast to modern songs which have a melody, usually sung or played by one voice, chords, and rhythm. This is what the theme of this fugue looks like:
In the first section of the fugue, each of the four voices introduces the theme, starting on a different note. In this section, the theme never overlaps itself. After all four voices have entered with the theme, Bach starts making things more interesting, and voices start playing the theme so that it overlaps with other voices. When the theme entrances overlap, this is called stretto. As you watch, you'll see that Bach uses stretto more and more, which heightens the tension of the piece.
The theme occurs 24 times in this fugue. That's no coincidence; there are 24 fugues in the cycle of the Well-Tempered Clavier (Book 1), two fugues for each of the twelve tonal keys -- once in major, once in minor (2 x 12 = 24). Bach loved numbers, symbolism, and symmetry, so this is very likely his way of showing that he intended to write 24 fugues altogether. See if you can hear each entrance of the fugue while you listen to Kimiko Ishizaka performing it.