Best answer: 19H1 is the codename that Microsoft is using for the development of the April 2019 Update, version 1903.
The development name doesn’t represent the final product name. Instead, it reflects the year and part of the year the release belongs. For instance, 19H1 represents the first Windows 10 feature update development planned for the first half (H1) of 2019 (19) . Usually, the first refresh update releases during the spring (April).
19H2 represents the second Windows 10 feature update development planned for the second half (H2) of 2019 (19) . Typically, the second refresh update releases during the fall (October).