They treated Russell's Studies in the Scriptures books as a replacement for the Bible, claiming that if one only read the Bible, they would 'fall into darkness' and be confused. But if they read SitS, they would understand the Bible and have a clarity that was not possible with just Bible reading alone.
To interpret the Bible is to find meaning in its words and to build ideas and concepts that are not clear upon a normal reading. This is why there are so many different denominations of Christianity (an issue that is not limited to that faith; pretty much every other religion has offshoots that disagree on the tenets as they understand them). JW leadership makes it clear that they interpret the Bible for the rank and file, and that it is dangerous for anyone but the GB to offer an interpretation, since they are getting their guidance from god, and you are not.