The point of this indicator is to determine which currencies as a whole, rather than pairs, are experiencing high volume.
It does this by drawing volume information from all 28 major pairs. It then takes two simple moving averages, one short-term and one long-term, and compares and normalizes the difference between these two (this difference is called relative volume). These relative volumes for each pair are then averaged together to find the relative volume for the entire currency. If relative volume is greater than 0, then the currency is experiencing higher volume than average as a whole. If relative volume is less than 0, the currency as a whole is experiencing low volume.
This can be used as a starting point to find specific pairs that are experiencing high volumes, or to determine which currencies are likely to undergo major changes in the near future.