The "kbps" number in an MP3's metadata, or kilobits per second, indicates the bit rate, or level of compression, that the MP3 encoder used to create the MP3. Roughly speaking, a higher bit rate indicates higher quality, so a 128-kbps MP3 is twice the quality of a 64-kbps MP3. An MP3 encoder can also create MP3s in either monophonic or stereophonic sound.
An MP3's bit rate indicates its level of compression. The higher the bit rate, the less compressed -- and therefore, the closer to the original audio -- is the MP3. A 128-kbps MP3 is compressed at a ratio of 11:1; a 64 kbps MP3's compression ratio is 22:2. This higher compression ratio means that the MP3 encoder removes more information from the audio while encoding the MP3. MP3s with lower bit rates are smaller in size; however, their audio quality also suffers due to the higher level of compression.
Mono and Stereo
A mono sound recording contains only one channel of audio. When you play a mono MP3 over a set of stereo speakers, the same channel plays on both speakers. A stereo MP3, on the other hand, includes both a left and a right audio channel, each of which contains slightly different audio. Stereo MP3s have a greater sense of space, while mono recordings can sound "flat." This difference is especially apparent when listening to music MP3 files.
Stereo MP3 files encoded at 128 kbps are, for most intents and purposes, close to CD-quality. At the lower 64 kbps bit rate, which has an audio quality similar to that of an FM radio broadcast, music can sound tinny or "thin." When you're ripping your CD collection to MP3, set the bit rate to 128 kbps at the very least and encode the MP3s in stereo unless you're certain that the source recording is mono. If you're recording a lecture or speech, however, a 64-kbps mono MP3 may sound fine.
You cannot increase the bit rate of an MP3, nor can you convert mono to true stereo. You can, however, save space by converting spoken-word, 128-kbps MP3s to 64 kbps or compressing stereo into mono. This process is called "transcoding"; only transcode an MP3 if you're willing to accept a substantial drop in its audio quality. Use a specialized audio conversion program, such as dBpoweramp or Oxelon, to convert a 128-kpbs MP3 to a lower bit rate; you can also use the "Import Settings" and "Create MP3 Version" features in iTunes to convert an MP3 to the lower bit rate. The converter dBpoweramp can also convert stereo MP3s to mono, if desired.