I guess this is marginally on-topic so I won't
delete your question, but I really don't want this thread to turn into a discussion of filler arc.
Filler is anything that is written for the anime which wasn't originally written in the manga. It is called "filler" because it fills in the time so that more manga can be written. Most (not all) filler is badly written and serves no purpose other than
filling in for a real story. A "filler arc" is a story arc consisting of nothing but filler.
For more information or discussion, please read:
Bleach news: Fillers ahoy. Further off-topic discussion will be deleted.