Anything that's not memorable, especially because it's just not very interesting, is forgettable. A forgettable meal at a restaurant might be bland, and a forgettable book may not have much of a plot.
When something is easy to forget and hard to remember, it's forgettable. This adjective is particularly popular with critics, as an easy way to convey how mediocre something is. You might have loved every single Pixar movie except for Cars, which you found to be totally forgettable. And while you're a fan of a band's first album, their recent one is simply forgettable — in fact, you can't recall a single song.