A filler episode, often called a "fillep", is a term created by the anonymous creator of the most-used Episode Guide to refer to episodes he added that may not have actually been present. These episodes are added in to help explain certain plot details that he can't extensively remember the plot for, or where exactly they originated from.
An example is Hi Allen. In the episode, nobody knows what happens, except that it spawned the phrase "hi Allen" that was used many times during the episode. The actual origin episode may have not been primarily about this phrase, and probably had a more full plot, but cannot be remembered clearly. Still, the detail is fairly important to remember and it thus given a "fillep".