MP3 is a computer file format which compresses music to “shrink” it to a fraction of it’s original size.
The idea is to compress the music in such a way so that quality is still kept intact. MP3 does this by throwing away parts of the music your ears can’t hear anyway (audio frequencies that are outwith the range of human hearing).
It also throws away very quiet pieces of the music which can’t be heard because of the louder parts drowning them out.
After it has performed some other tricks to the audio, you are left with an audio file which sounds exactly the same as the original, but is up to one tenth of the size. For a more detailed explanation, see my article on What Is MP3 – Explained.