When explaining the general meaning according to the sutras and the common teachings, "wisdom" means knowledge (prajna) which is primordially free from defilement, the basic state of emptiness. The word "essence" or hridaya means "extract" or "supreme," and is therefore compassion, the supreme path which is indispensible for accomplishing buddhahood. The Five Hundred Thousand Scripture says:
From the wish-fulfilling tree of emptiness
Appear the fruits of nonconceptual compassion.
This is the root of all the awakened ones.
From this they emerge, there is no doubt.
...a statement of explanation by H.H. Dilgo Khyentse Rinpoche:
"From emptiness endowed with the supreme of all aspects, represented in the form of the wish-fulfilling tree which encompasses all realms, appear the fruits of the twofold welfare (the welfare of self and others), the nonconceptual compassion, which is not a nihilistic void. This is the root which is the source of the kayas and wisdoms of all the perfectly awakened ones. From this unity of means and knowledge, emptiness and compassion, these emerge, there is no doubt." JOKYAB
(from the book Light of Wisdom, vol. 1, pg. 28, et al). [RY]
Therefore related to what's given above from the general meaning according to the sutras and the common teachings...Wisdom Essence means the "supreme path of emptiness endowed with a core of compassion."