I can't remember what they're called exactly, but you have to complete certain challenges during missions. For instance, it might mention that Ezio defeated some Guards without suffering any damage, so you have to defeat the Guards without being hit.
Things like that unlock the repressed memories I think.
Of course, if you've done this already and you're asking how to play them... Maybe they'll tell you during the game itself.
I am the true essence of Evil's bane. I am the Fierce Deity and I have risen again to kick your ass