An iPod is a small music player made by Apple. You could put a huge number of songs on it — from a computer, from a CD, or from another iPod — and carry it in a pocket or bag. Later kinds could also hold videos and games. For a stretch of years in the 2000s it was the fashionable way to walk around with your own music. Though actually, Apple has stopped making iPods; a phone now does the same job, and many younger people have never owned one. It is a brand name, not a general English word for every player, even if some speakers use it that way, the way some people call any tissue a Kleenex. Write it with that capital P.