The waiting loop is a premium feature and part of Travian Plus membership
This feature enabled any tribe to put one additional building OR resource field into the construction waiting loop as long as you have enough resources to do it.
After the first assignment is finished the next assignment in the waiting loop will begin.
Research Waiting Loop
This feature is similar to the waiting loop and allows you to queue additional research in the smithy
The queue will also be finished instantly if you use the gold feature Finish construction instantly