Join Shapes (Arrange Menu)
Join Shapes joins all the shapes and lines in the selection to form a single object. Because overlapping parts of the same shape are drawn with no fill color (you can see through the overlapping parts), you can use this effect to create holes in shapes, by joining several shapes together.
The Join menu options tie together a number of different shapes into one shape making all the parts that overlap see-through. For example, joining an object with a smaller circle on top of it produces an object with a circular hole in it.
Breaking lines and shapes apart
Creating objects with holes