From SC2Pod
Multiple Building Selection (abbreviated MBS) is a game mechanic allowing you to select more than one building at a time. MBS did not exist in the original StarCraft. When multiple buildings are selected, choosing to build a unit only makes one of the buildings begin training the unit. For example, if you have five Barracks selected and click the Marine icon, only one Barracks will start producing a Marine. To have each Barracks train a Marine, you must click the Marine icon five times.
Also if we select a group of workers they can build multiple buildings. For example if we have 5 SCVs we can build 5 Barracks pressing "B" 10 times ('B' for basic buildings , 'B' for barrack, selecting a valid emplacement five times this the routine for the 5 SCVs)
