I would say filler is a song on the album that has already been covered by other songs and is repetitive or redundent to the theme that has already been explained. It's a song that lacks quality because little effort was put into it and it's a song that adds no further depth to the album. It's fluff for all intensive purposes.
+4855555555555555555 I ****ing love that song, and it will be a single, Rated R had RudeBoy, Loud had S&M, RMO is the dirty anthem of the TTT it has to be a single.
Bad Kids is filler. And Fashion of His Love, but that's a bonus track so w/e.
I'm not saying they're bad songs... in fact, they're both considerably high on my list when it comes down to ranking them in their respective albums. They're just filler songs.
I don't like "Trouble For Me" in the way most do, but even I know it's not a filler. Femme Fatale is in the same boat as "Talk That Talk" 'cause it's all about selling sex; only song that doesn't fit Femme Fatale is "Trip to Your Heart". Same with "Talk That Talk", those cutesy songs i.e. "We All Want Love" & "Trip to Your Heart" don't belong in a sex album...
What's filler? Even We All Want Love isn't filler, no matter how little it compares to the rest of the album.
Birthday Cake is definitely filler. No matter how good the song is (it's high up on my list) it's not good enough to be a single. It's only on the album because it wants to be, so it's filler.