CBR = Constant Bitrate
In CBR coding, the bitrate will be the same for the whole file. It means that each part of the file will be using the same number of bits (if bit reservoir is not considered). Regardless of whether the musical passage is easy or difficult to encode, the encoder will use the same bitrate, so the quality of your file is variable. Complex parts will be of a lower quality than the easiest ones. The main advantage is that the final files size won't change and can be accurately predicted.
Very often CBR doesn't still mean exactly constant bitrate. For example, MP3 uses so called bit reservoir to give a small amount of additional bits for use when needed. However, CBR mode is very close to constant bitrate.