What does the music term rubato mean?

It is the Italian word meaning 'robbed'. It's an important element of a lot of music that is easy to hear, but more difficult to do well. Rubato means that there is a fluctuation in speed in the performance, some notes or phrases being played more quickly, and other notes or phrases less quickly. One way of thinking of it is that using rubato in a piece of music will give a performance of close to the same length as if the piece were played with a strict metronomic attack. The 'gives and takes' in a rubato performance balance out in the end.