Where do u guys get that? I know it sounds stupid but is it a book or..? I have seen many things about the "Chapter 847" but i don't know what is it. Is it an episode? (Sorry if this sounds ridiculous)
It looks like you watch the anime. You see, the anime is mostly based on manga. The anime sometimes adds some filler episodes or material to lengthen the series and give the mangaka, Oda, more time to write the original material. Chapter 847 happens to be the latest chapter of the manga on which the anime is based. As a result, the anime always lags behind the manga.
Oda himself writes(/draw) the original manga... then based on the manga they make an episode... currently the latest manga is Chapter 847... in the anime theyhavent left zou... but in the manga have already... (oops spoiler)... its up to you to decide which one to do or you can do both (like me :) )