Microsoft today rolled out new Cumulative Update KB3156421 for Windows 10 PCs. This is the new May cumulative update for the current Windows 10 Version 1511 and contains quality improvements and security fixes.
There aren’t any noticeable differences that you’ll see in the UI or new features. However, it does fix bugs and improves security settings under the hood. This is update comes after April’s update KB3147458.218 which also had bug fixes and security updates.