This achievement is for completing the game, and then watching the credits all the way through.
Just to warn you, a number of individuals (Sufi628 and myself) have had problems with this acheivement in that it has not unlocked at the end - however, for both of us all that happened was 1-2 days later, it suddenly unlocked, and was listed as achieved (it "popped" when I was not on my console.
I had replayed a couple of chapters during the game, and not always just clicking continue, so to give you the heads up, this does not affect the achievement unlocking, but just bear in mind.
So if you watch it all the way through and nothing pops, don't initially panic - as it will probably unlock when you turn your console off and are away doing other things!