Don Giovanni was a philanderer, preying on innocent young women, trying to seduce them. The opera starts with him trying to seduce, or if needed, rape the daughter of the Commendatore. When he is stopped by the father himself, he kills him and escapes.
After many other wrongdoings, his fate catches up with him in the shape of something that he cannot kill or seduce: the ghost of the Commendatore as a stone statue, knocking on his door. Don Giovanni is given a final chance to repent, but he refuses and he cragged to hell by his demons. The opera then ends with a moral message: the death of a sinner always reflects his life.
Don Giovanni is seen by many as Mozarts greatest opera. I have never seen it, but I cannot escape the scene in Amadeus where Mozart is visited by the same statue in the shape of his father. That is Don Giovanni for me.