He told me I have an innocent face, what does that mean?

Recently I matched with a guy on tinder. His opening line was "Innocent face" I replied back "I hope that's a compliment" he said, "of course it is". He has said it again to me recently on snap chat but I have no idea what he means by it and I don't want to sound stupid asking him! Can someone please tell me what an innocent face means?
He told me I have an innocent face, what does that mean?
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