Today, Microsoft announced the Windows 10 April 2018 Update, and it will be available on Monday. Insiders have been testing build 17134 for about a week and a half now though, so it should come as no surprise that the company is releasing a cumulative update for those that are already on the RTM build.
This is very common shortly before a feature update. One of the things that Microsoft has to do when testing a release candidate is test servicing, so KB4135051 is Insiders on build 17134.1, including those on the Fast, Slow, and Release Preview rings.
Microsoft hasn't published a changelog to list what's been fixed in today's cumulative update, and as the tweet above says, it's a very fast download and install. If there are fixes included in it, there are very few.
Of course, while Monday is the day that non-Insiders can manually install the update, May 8 is the day that it starts rolling out over Windows Update. If there are some fixes that you're waiting on, you might want to wait until then. After all, May 8 is also Patch Tuesday, so there will be an additional cumulative update then.
Update: Some users are reporting that this patch isn't arriving on their machine unless they re-enroll in the Insider Program.