Fillers are episodes that appear in the anime but not the manga. People often hate them because they are boring and almost always irrelevant to the manga plot (which is canon/official). The manga is controlled by Kishimoto, but the anime is not, which is why filler arcs tend to have terrible, pointless stories. Fillers are used to the anime running for a longer period of time and prevent it from catching up to the manga too quickly. To my knowledge, there have only been 2 characters introduced in fillers that eventually became canon - Raiga & Utakata. Although most filler arcs are despised by fans of the series, there are 2 that well liked - the 12 Guardian Ninja arc (which gave Asuma a background story and depicted Naruto trying to master Wind Style) and the Shadow of the ANBU arc (which expanded on Kakashi's background by showing what he did after Obito and Rin died, and also gave more background to Itachi, Shisui, and the Uchiha massacre, as well as established a connection between Itachi and Kakashi that existed before the series began).
