This phrase is often used in the political arena. It means that the best way to root out corruption is to expose everything, to be completely transparent.
I was reminded of this phrase recently when meeting with a client and their AMS vendor. We were meeting to discuss how to improve relations between the two groups. I made the point that the best way to ensure to continue a good relationship is for both groups to be totally honest and forthright about how things are going. You have to talk about both the good and the bad.
Put another way, if there are real problems between you and your vendor, bringing those problems to the attention of the vendor is the best way to get them addressed. It may be the only way.
So if you’re having trouble with your vendor, your staff, or anyone for that matter, try a little sunlight.