Learn Dutch: Wat ertoe doet, is het innerlijk. - What matters, is the inner self.

This is a pseudo-cleft sentence, which is used to place strong focus on the complement (het innerlijk). The entire subordinate clause Wat ertoe doet acts as the subject. The comma separates this long subject clause from the main verb is.

Where does the meaning "matters" come from in ertoe doet?

It comes from the fixed expression ertoe doen, which translates to "to matter" or "to be relevant." Literally, it translates to something like "to do to it."

Why is ertoe written as one word instead of being split up?

Ertoe is a pronominal adverb (combining er and toe). While it often splits in a main clause (like in Het doet er niet toe), the two parts stick together here because it sits in a subordinate clause where the verb (doet) is pushed to the very end.

I recognize innerlijk as an adjective. Why does it take the article het here?

In this sentence, innerlijk is used as a noun meaning "the inner self" or "the inside." When you turn an adjective into an abstract noun, it always takes the neuter article het.