The _across_masters suffix just averages the value across all HBase Masters running in the cluster. The _across_roletype generally applies the same way to all other role type metrics in CM. You can also query them without the across suffixes.
The total_ prefix sums instead of averaging the value, across all relevant role types for your chosen metric.
The rate bit basically appears for all counter style metrics. It represents the change over time (dx/dt). You can perform an integral(…) over any rate-named metrics to access actual value growth.