Sometimes after putting a design together you decide that it might benefit from some rounded edges. Instead of deleting the rectangle in the design and redrawing a new rounded rectangle to the proper size and position you can simply run a filter on the existing rectangle and get a rounded rectangle. Go to Filter > Stylize > Round Corners. This filter allows you to add rounded edges to the existing shape. When most think of rounded corners it is usually for rectangles. But rounded edges can easily be added to anything that has a point, so a polygon like a octagon or a triangle can be rounded just like a rectangle would.
The filter is a good method, but it is a destructive process, meaning that it cannot be edited after it has been applied. For a non destructive version of the filter go to Effect >Stylize > Round Corners. The result will be the same except the instance of this effect can be edited with the Appearance panel. It also can be applied to any shape that has a point.