When you create a new map by going to File | New, you are presented with two options to create a map inside UDK: additive and subtractive. The most common and mostly used is the Additive mode.
Think of an empty space. In the additive method, you would be adding walls, models, and the environment assets into the empty space. A real life example of additive map creation would be constructing a building wall-by-wall.
Think of a solid rock. In the subtractive method, you would be cutting the rooms and environment out of this solid mass. Subtracting the solid rock, in order to have room, is like sculpting.