There is so much that can be said about J.S. Bach! He lived from 1685 to 1750 during what we now call the Baroque era in Germany. He was a composer, organist, choir director, and conductor, and the list of everything he has composed includes 1126 compositions, including long works like the Goldberg Variations that takes 50 minutes to perform. He's also well-known for the number of children he had, which was 20, although only 10 lived into adulthood.
The piece you are about to hear is from the Well-Tempered Clavier. That's a little like saying "the well-tuned piano" except that they didn't have a modern piano, but did have harpsichords and clavichords. He wrote a prelude and a fugue for every major key and every minor key (that's 24 keys, so they are sometimes referred to as "the 48"), called it Book 1, and then did it all over again in Book 2. What is a fugue? So glad you asked. A fugue ("few-g") is style of composing where rather than having a melody and harmony, what you have is two or more (up to 5, sometimes, and 3 in this one you're about to hear) melodies or voices, and it's the way the two melodies interact with each other that create the harmonies, a little like singing "Row, Row, Row Your Boat" in a round. I know that often the videos are not particularly great because it's the music I want you to hear, but this video you're going to want to watch! You'll be able to see the melodies in the lights, which should help you to also hear them. Ready? Here comes the Prelude and Fugue in C Major from Book 1.