Musica ficta refers to the practice in medieval and Renaissance music where composers would use pitches that were not notated in the original musical score, usually to create smoother melodic lines or to avoid dissonance. This practice involved raising or lowering certain notes to improve the overall harmony and expressiveness of the music. Musica ficta is essential for understanding performance practices during this era, as it reflects the musicians' interpretations and the stylistic preferences of the time.