This is a warning that appears when you perform an OpenGL render. Because the OpenGL render is using your video card, letting another window open in front of the viewport (because you check your e-mail while waiting, for example) can confuse the system and will probably result in a partially or wholly black image. You can, if you like, use the check-box on the warning to stop it coming up in future, or you can leave it as a reminder.
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