Stacking refers to situations in which multiple ground units or buildings occupy the same space.
Generally, buildings can only be stacked when pre-placed in a third-party map editor, or when using certain trigger techniques. Most ground units can only be stacked in-game when a powerup is moved on top of them. Terran SCVs and Protoss Probes can stack themselves by constructing buildings over themselves; SCVs may also end up being stacked when completing a building. Ground units, when stacked, will shift around randomly in an attempt to un-stack themselves.
The method used to stack depends on what is being stacked (building or unit), and when it is being stacked (before or during the game). Stacking buildings tends to be easier.
What will I need to stack?
How do I stack buildings?
To stack pre-placed buildings, simply enable your editor's stacking feature, and place the buildings on top of each other. (In SCMDraft 2, this option can be found in the menus, at Options > Units > Allow Stack.)
There are two different ways to stack buildings during a match. The first requires that you choose a location for the stacked buildings, and stack a group of other buildings at that spot. During a match, when you move or remove one of those stacked buildings (via triggers), you can then move another building to that location; it will take the place of the removed building and stack with the others.
Alternatively, you may use the "lightswitch" method. If a player loses vision with themselves (via triggers), they can build structures "in the dark", and these structures will stack with any pre-existing buildings. (To turn off Player 1's vision with themselves, for example, you would have them run the AI script "Turn ON Shared Vision of Player 1 with current player".)
How do I stack units?
Unlike buildings, pre-placed units will not be stacked when a game begins (with burrowed units being the sole exception). Only pre-placed Unit Sprites can stack.