If you think essentializing is a positive thing, I suppose it means something like "to express or formulate in essential form". A sort of variation - and verbification - of Platonic forms.
In the post-modern, post-structuralist world, however, essentializing has negative connotations, and means something like "to engage in a misguided attempt to reduce something to its supposed essentials; to ignore contradiction, ambivalence, and dichotomy".