Hi,
'r' in this case would be the distance between the phase center of your antenna and a point in the far field.
Note that in the far field, electric field falls off with 1/r, so when you multiply by r, the entire quantity r*E is no longer dependent on r, but instead just on phi and theta.
The practical utility of this far-field quantity is that it can be divided by R in meters to get the E field strength at a point in the far field (useful for a rough idea of things like 3m emissions).
The relationship between r*E [V] and E field (V/M) is similar to the relationship between EIRP [W] and power density [W/m^2]. Note that both are a function of the particular input power being applied to the antenna.
Hope this helps,
--Eric