The master builder is a premium Gold Club feature that allows you to queue additional building constructions even if you do not have the resources available, It costs 1 gold each time to use and is billed at the start of the construction.
You will need to construct a building with the master builder if you don’t have enough resources to construct the building or your workers are already at work. The master builder waits till you have enough resources, or your workers have the time to construct your building.
In this example, the master builder was used to upgrade a Warehouse to Level 20, there are not yet enough resources, but the construction can be queued. It shows when the construction would be started provided no resources are used or removed during that time.