This shortcut will help you in many different instances in Illustrator when you need to repeat a transformation. I use it most when I am trying to repeat a shape over and over again at an even interval.

First create a shape or a stroke, and then transform it in some way. You can find the Transform menu under Object > Transform or you can simply right click on an object and go to the Transform menu. Make sure that you are either holding alt (if you are transforming freehand) when you transform the shape, or using the copy button in the transform menus so that you get it to copy your original shape.

Then press Ctrl + D and the transformation will repeat itself and create a new shape.

This shortcut can be particularly helpful when creating Spirograph type shapes or a sunburst shape:

I also use it when I am trying to create many copies of a shape for a clipping mask, or with the Transform Each function to create even more dynamic transformations.