Music is a constant game of questions and answers. You can hear a question in a chord, and eventually it is followed by an answer. For every chord there is an answering chord that belongs to it. A pianist can start a piece with one note, go around the world musically, but the fact that he ends in the same note, gives closure to our minds. Try it, it really works like that.
The easiest pieces of music are those where the composer immediately gives in to that demand. You want note A? You get note A. That makes the tune satisfying and easy. More complicated music plays with this, teases the listener. Modern composers sometimes don’t give in to the demand at all, leaving the listener with a question, but no answer. Did this piece end already? The lack of satisfaction, the feeling something is missing, is exactly what the composer wants to tell you.
Wagner wrote the long work in between 1857 and 1859, premiering it in 1865 in München. It is considered a masterpiece, well known for its use of harmonic suspension: the unanswered question. Wagner starts the work with a dissonant chord in the prelude, only to answer that chord at the end of act 3, hours later. In the meantime, he keeps exposing the listener to series of unanswered questions, leading to a work of tremendous musical tension.
The influence of this work cannot be overstated. The early modern composers Mahler, Strauss, Schönberg and Berg were all inspired by the ideas in it. Of course there were more reasons to experiment with tonality and harmony, but certainly in this kind of drama, Wagners influence was everywhere.