Filler material is any and all scenes or episodes that appeared in the Dragon Ball and Dragon Ball Z anime but that are not present in the original Dragon Ball manga. Whether filler is non-canon can be difficult to pinpoint, depending on whether the events are simply referred to later, incorporated after the fact or how much involvement the original creator had in its production. In general, filler provides for new stories without seriously upsetting the status quo.
Sometimes evidence or flashbacks are added into scenes that were shown in the manga as well (such as Gotenks' fight with Super Buu at Kami's Lookout, when Piccolo refers to the Garlic Jr. Saga). Filler is found often in Dragon Ball Z, as a means of not getting ahead of the manga, as they were both originally produced in parallel with one another. For example, the Garlic Jr. Saga was likely added to the anime to due to the Trunks Saga not having been fully released in manga at the end of the Frieza Saga anime.