Rubato (from the Italian meaning 'robbed') is a style where the strict tempo is temporarily abandoned for a more emotional or expressive effect. This rhythmic give-and-take in performance applies within a limited unit of the time-scheme of a composition – that time unit being the phrase. A lingering or hurrying over some note or notes earlier in the phrase is compensated by a corresponding hurrying or lingering over others later in it, and if this is artistically carried out a keen-eared listener following the beat of a metronome would find that the departure from the beat earlier was corrected later by a return to it. A feeling of freedom, rather than metronomic regularity, is given to the performance.
It's sometimes stated that whilst rubato occurs in the upper parts of a composition the bass should go on its way with undeviating regularity. This doctrine first appears in the eighteenth-century musical literature and continues to appear in the nineteenth in accounts of Chopin's playing of his own music and his teaching. It's obvious that such a doctrine is of limited application if serious harmonic clashes are to be avoided.
The use of rubato, if it's to sound natural, spring from a genuine or cultivated musical feeling; otherwise, instead of adding to the beauty and emotional effect of a performance it will produce the effect of discomfort in the mind of the listener.