In the game you will come across buildings that will trigger an alert from your Animus 2.0 database. Some of these buildings' profile pics will have a strange red eye symbol on the top left corner with a note from Rebecca saying you should investigate.
The goal is to find a glyph that's more evident with your Eagle Vision (which is nicely upgraded this time around to let you move as you view). Look around the walls of these buildings to find a red glowing symbol.
NOTE: This red will be a lighter red than the ones for enemies so don't get them confused! Learn to recognize the shade or you'll be running into unwanted company.
Once you're next to it, the Animus will prompt you to scan with Y button and you will enter a Subject 16 puzzle. The first time around as intro, I don't think you need to press Y. The game will proceed for you with conversation between the techies and a recording from 16 detailing what needs to be done.
You will have the option to come back to the puzzle 16 gives you, I think, but I always attempt the puzzle first and have got it both times I tried.
If you solve the puzzle as well, you'd have gotten both this achievement and A Piece of the Puzzle as well. And on your way to gaining some insight from your Animus predecessor...
Have fun! And good luck!
Update: Confirmed. You can definitely come back to the puzzles as expected but conveniently enough, besides going back to the glyph, you can also just enter your database under The Truth where each puzzle is stored to be solved at your leisure. :)