A projection is a new fragment, for which new grid cells are created and added to the part. They are inserted to fill up the space between the selected fragment and the end of the grid in the chosen direction. You can create projections along the +/- directions of each X, Y, and Z axis. This function is often used to create supports for the production of the part. If you intend to do this, project towards minus Z.
If the Overwrite Grid Cells of other Fragments check box is enabled, other fragments, or parts of other fragments within the section of the new fragment, are overwritten.