This takes effect starting with the line you are on when you type it. If that is not enough margin reduction for you, just type several consecutive control-D characters to get the amount you want. The first of these reflects the primary problem in using autoindentation. In addition, though, the value of this option also determines where your left margin will land when you go back part of the way to your window or screen's left margin. If the margin is presently in the eighteenth or the twenty-fourth column, the effect would be the same.
Many programmers reset that value to four.
This reduces line wrap in program source code with many levels of tab indentation. But you just might be creating a problem by doing this. With a shiftwidth value of four and a tabstop value of eight, though, there will be times when a control-D requires the editor to remove one tab from the whitespace sequence with which it starts each line, and simultaneously add four space characters. A few versions of the editor cannot handle this complexity in some circumstances, and will at times put garbage in your file. Even more likely is that the editor will mess up when it encounters tab characters in the middle of lines.