Pretty much all Windows Insiders got some love today. Along with Windows 10 build 17123 coming to the Fast ring and build 17623 coming to Skip Ahead, there's also a new build in the Slow ring. After over sixth months of Redstone 4 development and two Slow ring builds during that time, build 17120 is the second one in a week. It replaced build 17115 in the ring, which was released on March 9.
So it shouldn't come as a shock that there are very few new features for Slow ring users. All that's new are some Windows Defender Application Guard improvements.
You also might recall that build 17115 actually had no known issues, so you'll want to know that build 17120 does have some. They're all related to the recent Windows Mixed Reality improvements though, so if you don't use a VR headset with your PC, you shouldn't have anything to worry about.
A new Slow ring build also means that there should be new ISO images that will be published, probably next week. It also seems that we'll receive a new SDK Preview sooner, rather than later.