It refers to Intellect, Discipline and Strength bars. I am sure you have noticed when you have certain gear equipped, your bars for each increase or decrease. The optimum build would be 12 full bars, hence the tier 12. Each piece of gear has certain stats that help you achieve that build. You would be surprised in the variation in stats from different pieces of gear. A piece of gear reaches tier 12 status when it's stats are pretty much maxed out. Destiny Item Manager plug in for google chrome shows you what tier each piece of gear is. But you can do this on your own. With the right gear, you should be able to fill 12 bars and then you can start spec'ing stuff for your desired build. Want your super faster than your melee, go for it. Or melee and grenades and a slower super? It is all up to you!! For 95% of the time I have played this game, I had no idea about this and must have deleted some really good stuff.

So, in order to reach Tier 12 you must have 2 of the 3 (intellect, discipline, and strength) at 300 points and 1 at 120 points. Each piece of armor has points in at least 2 categories, some have a majority 1 category, usually exotics and artifacts that may have double intellect, or double discipline or double strength. Perfect Helmet will be a 46/46 Perfect Guantlets will be a 41/41 Perfect chest piece 61/61 Perfect boots 56/56 Perfect class item and ghost 25/25 Artifacts confuse me so someone else will have to give you that. Look at my characters. You will see they all have 300 300 120 for Int Dis and Str. That's Tier 12. Intellect determines the amount of time it takes to gain your Super without any additional help through kills or revives or orbs etc. having your intellect at 300 means you have the lowest cooldown for your super (sans transcendence or illuminated etc) Discipline determines the amount of time it takes to regenerate your Grenades once thrown without any additional perks like scavenger or the warlock Void scatter thing. Having your discipline at 300 gives you the lowest cooldown. Strength determines the amount of time it takes for your melee to recharge once used, again max is 300 and that's the lowest cooldown without perks like perpetual charge or the hammer Titan one that recharges it instantly on kills. The idea is to have 2 of them at 300 and 1 at 120 to ensure you have the lowest amount of cooldown time for those as possible. I should have also included the fact you can get Tier 12 by getting a 4-4-4 roll through int/Dis/Str as well.

They should have put the potential max in the game. I believe that's the part that confuse people the most. They get an item but they have no idea how good it is until they've puzzled together a set and notice that they're 60 points short somewhere. It would have been an incentive to know the potential too because most people probably don't even know that they can reach tier 12.