Once you've completed all twelve of Assassin's Creed 3's main story Memory Sequences you'll unlock a new side quest called Animus Synching. This can only be played online and appears as an additional menu option on the map screen.
The objective of this side quest is to place Pivot markers on the maps and use them to triangulate the position of new Pivots. There are ten of them in total to track down, and each Pivot located unlocks a new cheat in the main menu. Pivot markers can only be dropped at your current location on the map by going through the Animus Synching menu, and once you've placed at least three of them here they'll join up and indicate a highlighted green area that contains the next Pivot.