Rubato is of course an Italian word from the verb "to rob." It means robbing time from one part of a measure to add it to another.
Prior to Chopin surely one sees accelerandos and ritards which are also not in strict time, but Chopin's use of rubato is stunning. A good player will rely on precedent as to how to construct a phrase with rubato notated. Of course it's open to interpretation. If a player is very good and knows how the piece has been played in the past, the assymetry of a measure with rubato will sound correct and natural. You can imagine that the aural tradition goes to the time 'ol coughin' Fred was playing live.
My ex wife went through a period of studying all the études, nocturnes, waltzes and some of the polinaises back in my page turning days in the seventies, and I can only conclude that many people reject the new with no basis other than a lack of sensibility. Chopins music is high art. It's no longer so new which may put all of us at an advantage as far as enjoying it is concerned.
In the words of Jean Sibelius: "You'll never see a statue of a critic."
