In the Callas blog an article entitled Understanding Overprint was published:
The overprint function originates from the PostScript world of the last millennium. This has allowed to overrule a fundamental principle of the PostScript graphic model. In PostScript, normally only the topmost object is visible. All underlying objects are completely omitted by the top object (in all separations). This often leads to so-called “white gaps” (white edges around objects) when registration problems occur in multicolor offset printing machines. To solve this problem, the old litho trick of overprinting was introduced in PostScript Level 2. This is often used, for example, to avoid white gaps with black text placed over a colored background.
The overprint functionality of PostScript has been adopted unchanged in PDF. Today in PDF, the same effect could be achieved with transparency. This would avoid some problems (e.g. screen display) with the (obsolete) overprint function.
In his blog article Dietrich von Seggern describes in detail how the overprint function and the special overprint mode (introduced in PostScript 3) work in PDF.