To avoid a replay of the problems faced by the Windows 10 October 2018 Update, version 1809, Microsoft has taken a very measured approach to the release of the May 2019 Update, version 1903, with both a long spell as release candidate and a much less aggressive rollout to Windows Update.
That rollout starts today. While you previously needed to be in the Insider Program (or have a source such as an MSDN subscription) to download and install version 1903, it's now open to everyone through Windows Update.
However, Windows users are unlikely to see the update automatically installed for many months. Initially, only those who explicitly visit Windows Update and click "Check for Updates" will be offered version 1903, and even then, they'll have to explicitly choose to download and install the update. This is part of Microsoft's attempt to make Windows Update less surprising: feature updates are offered separately from regular updates because feature updates take a long time to install and regular updates don't (or at least, they shouldn't). This installation experience requires the use of version 1803 or 1809, and it also requires the most recent monthly patch, which is also released today.