Those of you who do not avail yourselves of fine Japanese televisual entertainment are probably unaware of the concept of 'filler' as applied in this context. So be educated!
Japanese animated television shows (anime) are, almost invariably, based on Japanese comics (manga). They usually start their run fairly soon after the manga begins. Each TV episode, as a rule of thumb, covers two issues of a manga. Manga are released weekly. Japanese TV shows are aired weekly. They also very rarely take breaks - they don't have clearly defined seasons, like American TV, or series, like UK TV. Generally speaking, they start, they run, and when the story ends, people stop watching, or something goes wrong, they finish.
If you were paying attention above, you will have noticed the logistical problem with this system. Anime go about twice as fast as manga. Combined with the fact they usually start, at most, a year (or maybe, possibly, two) behind the manga, this means if they just followed the manga, they'd run flat out material within a year or two of starting their run. Shows can do this, and then take a break while more manga gets written. This loses your continous audience, though, and due to the way Japanese TV works, there's no guarantee you'll get back on the air. So they don't generally do that.
Hence, filler. To slow down the progression of the manga's story in the anime, filler episodes are written. These are generally slipped in between the major 'arcs' of the manga story (although some shows slip in filler episodes during major story arcs...) and are very simply episodes which don't come from the manga plot, written (usually) by jobbing writers who often specialise in it.
Filler episodes are rather like those novels that occasionally get written as prequels / sequels / in betweens / different perspectives on famous novels, usually because the author (or author's estate...) is getting a bit hard-up. Like that kind of novel, they're extremely hard to write, usually fall into fairly predictable categories, and almost all the time aren't very good. I do feel for the filler authors, because you somehow have to write an episode (or an entire arc) which is exciting, engaging, somehow significant but also can somehow have taken place entirely in the background from the main story, which when it starts up again will roll along more or less without acknowledgement of the filler ever having happened. (The people who adapt the manga for the anime are not the same people who write the filler, and they rarely pay any attention to the filler writers' work. Filler also often seems to be drawn by different people from the main story arcs; it seems that apprentice animators get to do the filler before they get to do main-story episodes. The quality of the art in filler episodes is noticeably poorer on most series). This usually leads to comedy episodes or backstory, neither of which are very easy to do well.
Sometimes you do get good filler. Naruto has done this well in the past, slipping in a couple of rather well-done joke episodes after huge climaxes in the main story. They were a change of pace, served the purpose of buying a couple of weeks breathing space from the manga, and made people laugh. Sometimes, though, you get really, really, terrible filler. Horrible filler. One Piece's 'rainbow mist' arc springs to mind. Sometimes, the whole time gap / filler problem can entirely sink a series. Prince of Tennis, which ran together with Naruto in a one-hour slot for a long while and was very popular, was basically sunk by a badly handled filler situation, and never managed to reach the manga's conclusion. This happens surprisingly often, and to an outsider it's amazing that the TV world doesn't just adopt a season / series formula. But it seems the viewers won't have it; they must have an episode a week, and until that changes, struggling young Japanese writers will be cutting their teeth on writing filler for the foreseeable future. The only series that manage to escape the problem are short series which are often based on manga that already finished, series which manage to start several years after the manga, or original series. It's an intriguing thing to watch, from the outside.