Google is constantly experimenting with new features, new products, and changes. It needs to test these changes with actual users, which is why some people are always getting new features ahead of everyone else.
But some things are too sensitive to test outside of the company. Luckily, with tens of thousands of employees, Google has all the testers it needs. What's more, they're bound by their work contract not to divulge the things they see.
Even so, some experimental changes leak out, most often because employees don't even realize they're getting an experimental version of the site or app or whatever Google product they're using.
This is why Google notifies employees that they're getting an experimental version and that they shouldn't share the things they see with people outside of Google.
Sometimes though, Google messes up and sends those notifications to actual users. In fact, plenty of people have been noticing a strange warning on YouTube: "Experiment: There may be confidential content in your search results. Please do not share outside Google."
This message is obviously aimed at employees and not supposed to get out. But, someone somewhere made a mistake and enabled the warning for regular users. The problem has probably been fixed by now.