By cd, what does that mean?

CD stands for "Compact Disc". It is a type of digital storage media, most commonly used for music and audio files. CDs are made of a polycarbonate plastic disc, usually 120mm in diameter, which is coated with a thin layer of aluminum. The data is stored on the disc in the form of microscopic pits, which are read by a laser in the CD player. CDs can store up to 700MB of data, which is equivalent to around 80 minutes of music. They are a popular form of media storage due to their durability, portability, and low cost.