"-tte iu koto" or "to iu koto" means "It/That means that..." Literally, it can be translated as "the thing called __".
e.g. Anata ga suki da tte iu koto.
It means I like you.
e.g. Kono shiken wa kantan da tte iu koto.
That means this exam is easy.
e.g. Dou iu koto desu ka?
What do you mean?