This is the Redemption achievement. You receive it for getting the best ending in the game. As you play you will get white flashes that are accompanied by an audio cue that sounds like dripping water.
You should try to do these achievements during this playthrough:
Patron of the Arts
Back to the Past
Each of these level specific achievements will get you a positive flash. You can also get them by listening to conversations, giving bullets to beggars and by exploring. Basically play as passive and like a nice guy. There is no way to tell if you are going to get this until the very end of the game. Also, do not kill
I unlocked this achievement on ranger hardcore and you can watch the entire video below, or go to the watch page which has timestamps of the chapters in the description.
Sashamorning added this link: http://metrovideogame.wikia.com/wiki/Moral_Points
it shows every moral point in Metro 2033 and Last Light. Spoiler warning.