in that; in that place; in him/her/it.
"The key to solving the problem lies therein, as no other factor could possibly explain the sudden shift in behavior."
In plain English: Therein means within that specific thing you are talking about right now.
"He found his missing keys right therein on the kitchen counter."
Therein comes from Middle English ther-inne, which combines the words there and in. It traveled into modern English as a single word meaning "in that place."