それに is a conjunction meaning "besides," "in addition," "moreover," or "what's more." It is used to add supplementary information that supports or reinforces what was just said. The added information typically goes in the same direction as the previous statement – if the first point is positive, the second is usually positive too, and vice versa.
それに typically appears at the beginning of a new sentence or clause, connecting it to the previous context. It is common in both casual and polite speech.
Unlike そして (which simply means "and then" and can connect unrelated events), それに specifically adds a point that strengthens or supplements the previous statement. Unlike しかも, which carries a stronger sense of surprise or emphasis ("on top of that!"), それに is more neutral and conversational.
It can also appear mid-sentence after a comma: