as a matter of fact; to tell the truth
Used to introduce a statement that reveals the real situation, often contrasting with what was previously assumed or said.
To tell the truth, I don't know much about him.
As a matter of fact, I was against the plan from the beginning.
A set phrase from 実 (truth, reality) + を + 言えば (conditional form of 言う, 'to say'), literally 'if I say the truth'.