This means you CAN NOT take some istock video, edit it together and then sell your video. You can not even give it away for others to use, not legally anyway.
The only allowed use is for the finished video to be a free promotional video or a free educational videos. You can not sell or charge for the finished video in any way. You can not transfer the license by giving the edited product to a client.
LEGALLY, YOU CAN NOT MAKE A PRODUCT TO SELL OUT OF STOCK VIDEO.
...