kernel, substance, core, center, centre, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, pith, sum, nitty-gritty (noun)

the choicest or most essential or most vital part of some idea or experience

"the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"

nitty-gritty (Noun)

The essence or core of something; the details.