On April 4, Microsoft announced the Windows 10 May 2019 Update, promising to be more thorough in its testing than its ever been before. Version 1903 was pushed to the Release Preview ring of the Insider Program - something it skipped entirely with 1809 - a few days later, and the feature update wouldn't be offered for general availability until late May, after a full month of testing.
Now, after nearly a month and a half, Windows 10 version 1903 is available for all non-Insiders. You'll need to be a "seeker", something that means something different than it did previously. For the last couple of Windows feature updates, Microsoft has just given it to anyone who hits the 'Check for updates' button in Windows Update.
Now, your PC will tell you there's a feature update is available, and you'll be explicitly asked if you want to install it. That way, if you want to check for cumulative updates, you can get them without worrying about getting a feature update that might cause issues.
There are quite a few things that are new in the update, and you can find the full list here. One of the most notable front-facing features is a redesigned light theme. You can now have a light taskbar, as well as make other elements light, and you can split the settings for the system and the settings for apps.
Another interesting feature is Windows Sandbox, which is limited to non-Home SKUs of Windows 10 version 1903. This creates a virtualized Windows 10 environment; it's pretty much a VM without having to take the time to create a VM.
You'll also find that you can uninstall a lot more of the system apps, including 3D Viewer, Calculator, Groove Music, Mail and Calendar, Movies & TV, Paint 3D, Snip & Sketch, Sticky Notes, and Voice Recorder. This is great news for anyone who doesn't want those things.
Windows 10 version 1903 is available to download now through Windows Update or with the Windows 10 Media Creation Tool.