This tool simply duplicates the selected elements, without creating any connections with the rest of the mesh (unlike extrude, for example), and places the duplicate at the location of the original. Once the duplication is done, only the new duplicated elements are selected, and you are automatically placed in move mode, so you can move your copy elsewhere...
In the Toolbar are settings for Vector offset, Proportional Editing, Duplication Mode, and Axis Constraints.
Note that duplicated elements belong to the same vertex groups as the "original" ones. The same goes for the material indices, the edge's Sharp and Seam marks, and probably for the other vertex/edge/face properties...