Estimated average production per month of all crops in the schedule. This estimate is based on the crop growth time and soil fertility equilibrium (or the target fertility set via the slider below). Losses incurred due to lack of fertility or water are not included in this estimate. However, extra time incurred by slots that have no yield is included. To see how much food is needed, check your food market consumption report.
The number of workers available. To get more workers increase your population. You can do so by building a beacon, explore the world map or apply edicts.
This rough estimate is based on the current production level. Idle times (such as due to a full output) are not accounted for, in which case the duration left might be greater.