partition-specific rate parameter

[rjavonil]

hi all,
i'm fairly new to bayesian inference in phylogenetics, so please be kind.
according to the mrbayes manual, we can invoke partition-specific rates using the command 'prset ratepr=variable.' the output (viewable in tracer) lists m{1}, m{2}, etc. as estimates of the rate (multiplier) for each partition.
i don't exactly understand this parameter, so would you nice folks please explain it to me? what units is it measured in?
thanks in advance.
rob