1. What sets the truly righteous — the tzaddikim — apart from the rest of us is that they consistently use their free will to its best end. They face the same moral and spiritual challenges we each do and they elect to do whats right, no matter how difficult, while we often dont. We can undo our poor choices, to be sure, and become righteous ourselves, but unless we do that as consistently as they, we arent righteous. We can be good, kind, accepting, and more but we wouldnt be truly righteous until we also prevail over those challenges.
Now, the great near-Divine benefit and distinction the tzaddikim enjoy is the fact that they act as G-ds partners in rectifying the world, as Ramchal puts it (see Shir HaShirim Rabbah 5:2). As such, they play a vital role in activating the various phenomena that will eventually have G-d bring about the perfection of all things. But that calls for some explanation .
2. That means to say that with their righteous deeds the tzaddikim are able to activate the various phenomena here on earth that allow for G-ds great blessings to emanate from Heaven that will set off the great redemption and the perfection of all things.
As each mitzvah they fulfill and each possible sin they thwart plays another small role in the process. And though the process is a slow, step by step, calculated series of strides, the whole of it moves inexorably onward and will continue to until it achieves its goal. Thats so because each step fosters a response from G-d, the other Partner” in the process, if you will.
Recall, though, that the whole process has been on-going since Adam and Eve erred in the Garden of Eden. Thus the tzaddikim are charged with first bringing the world back to the stage it had been had Adam and Eve not sinned (which is a laborious task to be sure), and then theyre charged with catching up, so to speak, which means to say with bringing the world to the stage it would have been in had Adam and Eve not sinned in the first place (as we’d explained before) .
Nevertheless, once all of that is accomplished the world will have been perfected, our people will be fully redeemed, and G-d’s sovereignty will be manifest — all thanks to the on-going selfless efforts of the tzaddikim and of G-d’s own will working in tandem.
 Whats interesting is the fact that Ramchal attributes this ability to the tzaddikim alone at this point of the chapter, while he attributes it to the entire Jewish Nation by the end of the chapter. That either implies that each and every Jew has the wherewithal to be a tzaddik (see Maimonides Hilchot Teshuvah 5:2 for that very point); that we each play a role in the redemptive actions of the tzaddikim (since, for example, they cant engage in full tephilla without a minyan which wouldnt necessarily be comprised of other tzaddikim; they can only be generous to the poor if and when there are poor, who wouldnt necessarily be tzaddikim, etc.); or it underscores the idea that since All of Israel has a place in The World to Come (Sanhedrin 10:1) then all of us are tzaddikim at bottom and that we need only decide to draw upon our inborn righteousness and it will manifest itself.
For this chapters Kabbalistic references see Klallim Rishonim 29; R Friedlanders supplementary comments on p. 172 of his edition; R Shrikis notes 134-135; and R Goldblatts notes 9-10, 13 as well as note 82 in his edition.
 See 3:14 and 4:11 for example.
Rabbi Yaakov Feldman has translated and commented upon “The Gates of Repentance”, “The Path of the Just”, and “The Duties of the Heart” (Jason Aronson Publishers). His works are available in bookstores and in various locations on the Web.