It means that it's a very difficult crime to report. The only case where you can report child pornography, is also a case where you can not prove it without put

It means that it's a very difficult crime to report. The only case where you can report child pornography, is also a case where you can not prove it without putting yourself in legal peril. If you take a picture of somebody looking at child pornography on a monitor, you could be charged with creating it. If you see it on a website, and report it, you could be charged with receiving (and possibly possessing).