Hero's Fortitude, Resolution and Vitality
Hero's Fortitude and Hero's Vitality as well as Hero's Resolution were all added in during Call of the Forsaken and act as achievements that further advance your character. Each expansion adds in more tiers of these rewards and each of the three, Fortitude, Vitality and Resolution do something different. Each rank of them isn't much but as you could imagine if you keep completing achievements and doing progression the rewards will add up to be something significant.
In order to unlock these achievements you must do a number of different things. Usually these upgrades are tied in with progression in each expansion as well as collecting all of the group task achievements. These are the two most common things you'll find yourself doing to raise your ranks of Hero's Fortitude, Vitality and Resolution. Below you can find a screenshot of one of the achievements involved in the TDS Progression chain. As you can see the reward for completing the achievement is a rank of Hero's Resolution.
The one in the screenshot above is frankly not too hard to get compared to doing the special challenges for each mission. Some of them can be extremely difficult to do with groups or while boxing. The screenshot below has an example of one of group missions special achievements you need to accomplish to unlock future tiers of these heroes achievements. Each of the three achievements take place in the Dead Hills Mission 1 Xulous Prime and feature a brief description on what to do.
Each mission for each expansion has achievements like this and successfully completing them all will net you another achievement which is usually the one that shows you which ranks of the Heroes bonuses you'll get for finishing all the special achievements from that X-pacs tier. If it's a little confusing just type /ach and click through your achievements for a bit, you'll eventually get how they're laid out and how they work.
Just because I decided to add a few more screenshots to show you what Hero's Resolution and Hero's Vitality do.