Flash content that's deemed to be not central to a page—advertisements and peripheral animations—will be paused automatically on visiting a page. To make the content work, it will have to be clicked first. Videos, games, and similar important content should work without changes.
Google announced an equivalent change to Chrome last year. Since September 2015, Chrome too has tried to pause non-essential Flash content. Google's argument was that this behavior would be much better for battery life and that stopping ads from playing would make the browser less of a power hog. Microsoft also suggests that battery life will improve, but the company is positioning the change as more of a standards-compliance issue. Microsoft says that there are now many standardized alternatives to Flash and that developers should continue to adopt these technologies and phase out their use of Adobe's proprietary platform.