What is prajna?

Capable of upvoting (praise) of what is or leads to essential, a mind directed to unbond: this is called pañña.

Tendency of downvoting (blame) of what leads or is essential, a mind directed toward views, stacks, exchange and trade: this is called lack of pañña.

Uncapable to trace the essence in the essential, seeing unessential as essence, how could a grasper of pointless ever benefit of even highest knowledge?

Those who regard non-essence as essence and see essence as non-, don't get to the essence, ranging about in wrong resolves.

But those who know essence as essence, and non-essence as non-, get to the essence, ranging about in right resolves. ✦

As a Khmer proverb, an advice, says:

យល់តូច កំុអាលខំ យល់ធំ កំុអាលស្រាយ

Understanding/knowing little,

don't brand (judge) things.

(even if) Understanding much,

don't brand (judge) as heartwood.`

...

The skill of a fool can only harm him; it destroys his merit and his wisdom (lit., it severs his head). story

So the Sublime Buddha did not pay much judgement in regard of wisdom viciversa knowledge but whether corrupt and a fool or not:

Got the essence or another death having searched for stacks, exchange and for gains where nothing would hold?

Be(come) wise!

[Note that this isn't given for stacks, exchange, binding trades in the world but for escape from the bond of fool]

Kusala (ku=bad, sala=cutting off/away) is doing which getting at least 2 values, now and then (6 values--present value, next value, nibbana, own value, others' value, each others' value. See the last quote.)

Pajānāna (Pa+ñā) is understanding the possible various angles/aspects, causes and effects -- realities, in each moment. It means not only value and not only emptiness.

The Buddhist Followers use every kind of Pajānāna to enlighten Nibbāna properly.

It is impossible to say "I am a genius man in emptiness but I don't like to give or mettā" because giving and mettā always arise with the Pajānāna of emptiness.

The real Nibbāna is teaching only in the Buddha's teaching because of the Buddha is teaching of understanding in the proper&possible path to understand emptiness, causes&effects. And the proper&possible path includes the basis which supporting to understand the emptiness, such as understanding karma, understanding mettā, understanding people, understanding the smallest elements, etc.

The possible reality can't esily arise without it's supporters.

This is why the Buddha teach everything included Samatha, 40 Kammatthāna, not only Vipassanā.

Ñāna is Proficiency Pajānāna. It's when you are knowing one, you understanding every aspect of that one together in same moment without using thinking or comprehending time requirement because you are professional of it's relations, causes and effects, already.

For kusala see AndhaSutta (blinder of 2 values):

These are the three people found in the world.

Neither suitable wealth, nor merit do they make. They lose on both counts, those who are blind, with ruined eyes.

And now the one-eyed person is explained. By methods good and bad, that devious person seeks wealth.

Both by fraudulent, thieving deeds, and also by lies, the young man’s skilled at piling up money, and enjoying sensual pleasures. From here they go to hell— the one-eyed person is ruined.

And now the two-eyed is explained, the best individual. Their wealth is earned legitimately, money acquired by their own hard work.

They give with best of intentions, that peaceful-hearted person. They go to a good place, where there is no sorrow.

The blind and the one-eyed, you should avoid from afar. But you should keep the two-eyed close, the best individual.”

Prajna is direct practical realization of Buddha-Dharma.

It is when you 1) clearly see in front of your nose (and everywhere around you) all that cool stuff being spoken about in Buddhist books, and 2) have attained actual hands-on mastery of these theories and principles in your own day-to-day life.

Another way to say the above, Prajna is when 1) you attained a whole new level of discernment, when various ethically important qualities of processes and phenomena are clearly discerned by you. And 2) you learned to put these discernment skills to good practical use.