Also, the chord in its context with the prelude and Liebestrod: https://www.youtube.com/watch?v=n4bqRlNSQQE
The central idea of classical music to that point was that there is a core note, the tonic, and it sounds good when you resolve that tonic by playing around it and returning to it. Wagner introduces the tonic in the prelude as his Tristan Chord. It's representing a love that can't be fulfilled because he's the knight of the king she's pledged to wed. Wagner spends four hours teasing the audience with an infuriating series of notes that never register as complete music. It's the first real modernist use of dissonance in music I can think of, using an unpleasant sound for a bigger reward.