Learn Russian: Это моё дело.

In this sentence, это is a fixed word that means "this is". When introducing or identifying something ("This is my business"), we always use это, regardless of the gender of the noun that follows. You only need to match the noun's gender when saying "this [noun]", like этот дом (this house).

Why is the word for "my" spelled моё instead of мой or моя?

Possessive words for "my" must match the gender of the noun they describe. Because дело ends in -о, it is a neuter noun. Therefore, we use the neuter form моё. For comparison, you would use мой for masculine nouns and моя for feminine nouns.

Does дело mean a commercial business, like a company?

No. While дело translates to "business" here, it means a "matter", "affair", or "personal concern"—exactly like the English phrase "mind your own business". If you want to talk about a commercial enterprise or shop, you would use the borrowed word бизнес.

How does моё relate to the personal pronouns we are currently studying?