If you press start and scroll left until you get to DNA TRACKER, it shows you a percentage. You get this achievement when that percentage reaches 50%, there is also one for 100%.
You get the percentage up by completing optional objectives in missions, finishing homestead missions, liberating missions in Boston and NY, finding collectibles(feathers, chests, almanacs, trinklets), unlocking parts of the underground, discovering forts, and crafting certain inventions for your homestead.
Also, all story missons as well as naval ones. Levelling your assassins count, delivery and courier quests, hunting club, brawling club etc. - Thanks to Lavindathar