I can't actually remember now ¬_¬ But! I think it's the accuracy of the bounce etc.
So with security on - if the ball hits the edge at 45.66 degrees it will bounce off at 45.66 and not 46 etc. and with it off it'll just be more random. Hope that helps, I haven't used that in some time now.
Yeah, I've always thought it was rather pointless, because I think that if you set the randomness to zero, it should have the same effect as setting security to 100. Odd.