Simply Tanya
צדיק וטוב לו צדיק ורע לו רשע וטוב לו רשע ורע לו ובינוני. ופירשו בגמרא צדיק וטוב לו צדיק גמור צדיק ורע לו צדיק שאינו גמור
Simple Tanya
The righteous person for whom it is good , the righteous person for whom it is bad. The wicked person for whom it is good , the wicked person for whom it is bad . The average person . The Talmud explains that the righteous person for whom it is good is because he is completely righteous and the righteous person for whom it is bad is because he is not completely righteous .
Even more simply
Here the Alter Rebbe is introducing who are the " five groups " mentioned early . They are really three that divide into five.
More so , he informs us that there can be various types of righteous and wicked people . He also takes the time to point out the reason for why the righteous suffer and the wicked prosper .
To the child
Even while still a child , know righteous person can have levels of righteousness and degrees . The righteous person might have areas that still need improvement . also though the wicked person might be completely wicked he yet may still have some good in him . Life as a whole is many times a mixture of good and bad . thus don't be discouraged if things don't seem to be working out to your liking . As a child you might wish to view the world as black and white , good or bad , that is true but not always is it identifiable as such . more importantly know that every good you do has value even if you feel the majority is bad , conversely every bad you do diminishes something, even if the majority is still good. Hence you must constantly strive regardless of what level you currently are at .
Not so simple
Why does the Alter Rebbe use this Talmudic source to introduce the Tzadik , Rasha and Benoni ?
Are there not other place ?
Why does he not explain the wicked person who prospers and the wicked person who suffers ?
Why does he not explain the Benoni ?
If he is prosperous or suffering ?
Also why the need to bring the complete quote of the Talmud explaining the the reason why a righteous person suffers .
Just showing how there are five various levels of people , should be sufficient ?
Also why does he not say where in the Talmud he is quoting , as he does sometimes in other places .
You cannot say its because he is quoting the Talmud hence he needs to quote it in its entirety . remember everything in the Tanya is exact , every letter the Alter Rebbe had a reason for it . also the Tanya being like Chumash can be taught to the five year old as well as to the scholar . The beginner and the advanced . The five year old student level knows chumash but not Talmud .
Is there something to be gained by quoting the complete text ?
Later he will quote the Zohar , why not conclude all text of the Talmud on the subject and then quote Zohar .
I think perhaps we can say that , first the Alter Rebbe is teaching via the Talmud that we may question Gd , not to discard our faith but to understand Him better . Hence , why do righteous suffer and the wicked prosper ? The Alter Rebbe then must explain that it is because they are not complete in what they are . the suffering or prosperity will complete them .
However once the Alter Rebbe does bring this up he needs to explain that incomplete righteous does not mean someone who simply is committing a sin Gd forbid in fact the evil is nullified to the good , the problem is that nullification of something implies there is a residue of it there . Thus the need for quoting the Zohar / rahya mhemna right away to make that idea clear . The incomplete Righteous person is free from sin only inside he has a nullified piece of evil . plus this is the entire basis of the alter Rebbe tanya , that the righteous and wicked we are dealing with is the state of nullification of evil and self nullification of the soul to Gd , not good and evil of thought speech or action but of the soul itself in its struggle of transformation .
Postscript
There are many levels to a person . many a time a person can be a mixture of good and evil. Be careful how you judge another for you do not who is completely righteous nor who is completely wicked . some one might suffer because they not righteous enough or for other reasons while another might prosper because it is to punish them . Hence judgment is left better to Gd . looking at yourself you also cannot be sure all the time who you are hence do not be arrogant or overly confident in your self on the hand . on the other , it might be best not to dwell to much on your inner self , to much soul search will leave you lost , and not give you the clarity you seek .