When you "hit it off" with someone (often in the context of a romantic date), it means that the social interaction was pleasant, enjoyable, fun; the two people in the situation quickly felt comfortable talking to each other and became "fast friends."

If you say that you 'hit it off' with someone, you felt an immediate connection with that person and got on well together. You and the other person felt that you had a lot in common, had plenty to talk about and enjoyed each others' company right from the start.

It means "to get along with it".