Transform Again is a tool in illustrator when you need to repeat a transformation.
First create a shape or a stroke, and then transform it. The Transform menu is placed under Object > Transform or you can right click on an object and go to the Transform menu. Make sure that you are either holding Alt when you transform the shape, or using the copy button in the transform menu so can copy the original shape.
Press Ctrl + D and the transformation will repeat and create a new shape. This shortcut is very helpful when creating Spirograph type shapes or a sunburst shape: