The web is a great vessel to spread information to everyone, but not all of that information is created equal. Between hoaxes, mis-informed articles, sensationalist headlines, mislead opinions presented as facts, there's a whole lot of wrong or incomplete data being given to people and not enough accurate and well-researched information.
Fact-Check websites and the International Fact-Checking Network aim to solve that problem by rigorously gathering all data and producing articles that are both thorough and accurate (or as accurate as possible given the details at hand at the time). And now these sites will have a visible label in news articles to make them stand out.
The "Fact-Check" tag follows "In-Depth," "Opinion," "Wikipedia," and "Local Source," as a news tag. It will show up on news.google.com and in the News & Weather apps on Android and iOS in the US and UK. If you serve fact-checking articles, you should check out the source link for what to do to add that tag to your relevant posts.