It's a tool provided by Microsoft. Details are here: https://www.microsoft.com/en-us/photodna

Nobody wants to download someone else's movie collection and find that there's child exploitation material buried in there.

It only seems to work with images. There are privacy concerns with using this because it is an online only service. I don't think you can get an offline copy of the signatures/hashes.

No one wants to see the horror of child abuse material but realisticaly aside from rumour I havent ever seen or heard from anyone whom this has happened to.
Running all material through microsoft simply means that we are all providing a record of our activiites for a court to demand access to.

Some might be happy with such self regulation but I am not and I dont remember anything in the licence about working for MS as an unpaid researcher.
If I download anything thats abuse related I know the address for the CEOP and will do my best to assist them in hunting down the scumbag hosting it.