The “bitrate” refers to the level of file compression in an MP3. The larger the bitrate, the less compressed it is.
Despite what you may have read in the papers, MP3s are not exact duplicates of the files on your CDs. Instead, the MP3 is a compressed file, and the type of compression is what computer-using folks term “lossy”, which means that information is “lost” when the MP3 is compressed.
For the most part, MP3s are designed to “lose” sound information you don’t hear anyway – CD Audio replays a large spectrum of sounds, even those human beings can’t hear. So the MP3 codec ditches some of those ultra-high and low frequencies. Bitrates are the deciding factor in just how much information will get ditched. The higher the bitrate, the more sound information is preserved. One of the results of this is what’s called a “data artifact” – if you listen closely to low-bitrate MP3s, you’ll be able to hear slight glitches, especially in the treble frequencies.
Thus, a 320k MP3 has more sound information than a 128k MP3. But, a 320k MP3 is also three times as large in megabytes as a 128k MP3.