MP3 is a way of storing music as a computer file. These files use a code made up of numbers. But MP3 files are much, much smaller than the ones normally used on CDs. MP3 files are easy to send across the Internet. They can also be stored on CDs.
Thousands can fit on a computer hard drive.
However, the music from an MP3 file is not quite as good as the ones normally found on CDs. But, for most listeners, the sound quality is good enough. A lot of people think so. Get the explanation here.