Mentioned elsewhere, brought to here.
Very simple: a button on all builder units/buildings that, when pressed, brings that unit to the front of the queue for resources.
So, if you have two constructors that each take -10 metal in the process of construction, and your income is +15 metal, ordinarily each would build at +7.5 metal (75% speed); if one is prioritized, it will build at +10 metal (100% speed) and the other will build at +5 metal (50% speed).