Bitrate is very important ...
It is simply the amount of file space that is used up per second ...
a bitrate fo 128 kbit is the same as 131072 bits per second ... or 16384 Bytes per second which equals 983040 Bytes per minute ... or just under a meg (960KB) .. for Mp3 ... a few years ago (around 1996 or 1997) 128kbit was the norm, because almost everyone was on 28.8 or 56k, and it took a very long time to encode stuff on that old p150 ... and the Encoders took a long time .. (i remember ripping cds at 1x with DAC and then encoding them to 128kbit with l3enc on my Cyrix p200+ ... it took about 8 hours to fully encode a 1 hour CD at 128kbit ...)
Since today many people have faster connections and faster CPUs ... the standards have also gone up for digitally compressed audio. Mp3s typically are ripped at 192 or even 224kbit ... (i cant tell the difference between them ... but some people claim they can) VBR is another common compression method ... I like the Variable bit rate the best myself ... because when there is no high pitch sounds ..there is no need for the High bandwidth .. and then when there is the need for extra bandwidth ... it kicks in ...
Also ... most 128kbit Mp3s sound great if they were encoded right. If some fewl ripped something with a skippy drive and then encoded it in xing .. it will sound like crap ... but if the person took the time to rip in windac or audiograbber ... and then encide with Fraunhoffer with high quality checkked at 128kbit .. it will sound better then many 160 or even some 192kbit rips