An MP3 is a computer file that holds music (or other sound) in a small package. The letters stand for MPEG Audio Layer 3 — a way of squeezing sound so it takes up less space, by throwing away bits most ears will not miss. In the 1990s and 2000s this is how people passed songs around on the internet, and an “MP3 player” was a little gadget that played these files. Though actually, a lot of what people still call an MP3 is not one any more: phones and apps often use other formats, or they just stream the song and never keep a file. People still say “send me the MP3” to mean “send me the track.” You say it as letters: em-pee-three. It is an ordinary tech word, not fancy and not rude.