Usually, this is something someone says to emphasize that what they have just said or are about to say is true. It is like an oath: "I swear by everything that is holy that everything I say is true."
However, we can't be certain that this is what it means where you saw it unless you give us more context. Where did you see or hear this? What was said before and after this? (You can give us up to 4 sentences, total.) What were they talking about? And so on.
Edit: Glamorgan posted while I was writing this. We agree, it seems.