The word "rub" in the context of Shakespeare's work and the time is synonymous with "problem" or "issue," so what it meant was - that is the source of the problem, or this is where the issue lies.
He never returned to his home-town, and therein lies a mystery.
therein
barren
The cast of Therein - 2012 includes: Yotum Almor Rebecca Doran as Juniper
26.2
Four.
16
1000
No. It is an adverb. It means "in that respect."
2
The word therein. They are: There I in the here her he rein ere herein and therein.