My mother had to explain that one couldn’t compose a Liszt rhapsody because it was a piece of music that Liszt himself had composed.
Meaning of the quote
The quote means that you cannot create a new Liszt rhapsody, which is a type of musical piece, because Liszt, the famous composer, had already written those kinds of rhapsodies himself. Prokofiev's mother had to explain this to him, since he had tried to compose a Liszt rhapsody, not realizing that Liszt had already done that.
About Sergei Prokofiev
Sergei Prokofiev was a renowned 20th-century Russian composer, pianist, and conductor who created masterpieces across various music genres. From his iconic compositions like ‘The Love for Three Oranges’ and ‘Peter and the Wolf’ to his groundbreaking ballets and operas, Prokofiev left an indelible mark on classical music history.