CDs are a physical media storing digital data. The data is stored in microscopic pits on the disc surface that, like all physical media, can be subject to read

CDs are a physical media storing digital data. The data is stored in microscopic pits on the disc surface that, like all physical media, can be subject to read errors, due to various imperfections. It has built-in error correction but it is not perfect, so that program reads the same thing multiple times to get statistical confidence that the data is correct.