Substance, purport, gist, burden, core, pith can denote the inner significance or central meaning of something written or said.
Substance implies the essence of what has been said or written devoid of details and elaborations; the term is used especially when such an essence is repeated for the sake of others, but it may be used also of what characterizes a discourse and gives it body as distinguished from the frills or rhetorical froth that give it finish.
Purport lays the stress upon purpose or intent but when used of written or spoken discourse it applies to what is intended to be conveyed or imparted and so actually refers to the central meaning. It is often interchangeable with substance but always with the implication of the speaker’s or writer’s purpose.
Gist refers to the material part (as of a question, an argument, or a discourse); it is the substance thereof reduced to its lowest terms.
Burden implies the part most insisted upon or most often repeated and usually means the main topic or theme.
Core can apply to various things that give the effect of being whatever remains after the outer or superficial part is stripped off; in application to what is written or said it emphasizes the centrality of the meaning and the relative unimportance of the other aspects.
Pith often equals substance in the sense of body ; actually, however, it implies substance which gives a discourse its concentrated force, vigor, or vitality and is, therefore, a narrower and more expressive term.