The pronoun it in the sentence
A controlling idea: What the writer is going to focus on it in the paragraph.
is unnecessary and ungrammatical.
Such pronouns, which are usually but not always in a relative clause, are called resumptive pronouns. This is a grammatical feature of some languages, but not English.
An exception in English is the informal resumptive pronoun in statements such as following, with the resumptive it:
This virus, it's such a tragedy.
Your sentence should be rewritten without the pronoun as:
A controlling idea: What the writer is going to focus on in the paragraph.
Alternatively:
A controlling idea is what the writer is going to focus on in the paragraph.
Wikipedia has this on resumptive pronouns in English and other languages.