LAME is able to encode your music using one of its 3 encoding modes: constant Bitrate (CBR), average bitrate (ABR) and variable bitrate (VBR).
Constant Bitrate (CBR)
This is the default encoding mode, and also the most basic. In this mode, the bitrate will be the same for the whole file. This means that each part of your MP3 file will be using the same number of bits. Whether the musical passage is a difficult one to encode or an easy one, the encoder will use the same bitrate, so the quality of your MP3 is variable. Complex parts will be of a lower quality than the easier ones. The main advantage is that the final file's size won't change and can be accurately predicted.
Average Bitrate (ABR)
In this mode, the encoder will maintain an average bitrate while using higher bitrates for the parts of the music that need more bits. The result will be of higher quality than CBR encoding but the average file size will remain predictable. So this mode is highly recommended over CBR. This encoding mode is similar to what is referred to as VBR in AAC or Liquid Audio (two other compression technologies).
Variable Bitrate (VBR)
In this mode, you choose the desired quality on a scale from 9 (lowest quality/biggest distortion) to 0 (highest quality/lowest distortion). The encoder then tries to maintain the given quality in the whole file by choosing the optimal number of bits to spend for each part of your music. The main advantage is that you are able to specify the quality level that you want to reach, but the downside is that the final file size is totally unpredictable.