The resource efficiency is a calculation that considers how many resources are consumed in the process of upgrading one of the resource buildings in the outskirts of a city. It is expressed in terms of time to recover the resources spent on upgrading the building.
For example, upgrading a cropland from level 15 to level 16 consumes 17,375 total resources of all kinds for this upgrade. No consideration is given here for any specific resource type. In order to recover the resources spent on upgrading this building, it will take 3 days, 17 hours, and 6 minutes with the increase of 195 additional food resources produced before a player will have produced all of the resources spent on upgrading the building.
From a strategy point of view, it is better (usually) to take the time to bring up the lower level resource buildings first before embarking on a strategy to improve a higher level building, as it simply is a better use of your resources to be concentrating on the lower level buildings.