How do Windows version numbers work?

1903: 19 refers to the year 2019 and 03 refers to the month. Similarly, last year in 2018 we had 1803 and 1809, so 1803 was scheduled to be released around March (03) and 1809 was supposed to come around September (09) but got delayed a lot because of the file deleting bugs and other issues.

Regarding the 17134, that is just the build number which you can check by + R, then type "winver" and it will display your version (1903) and your build number. Two people might be using the same Windows 10 version (such as 1903) but could have different build numbers based upon their most recently installed Windows cumulative update.

It is, rather boringly, year:month

It's slightly harder to figure out because they don't always manage to hit the correct month they intended.

From Gaijin.at - Windows Version Numbers

Name / Description          Version Build   Public Release
Windows 10, Version 1507    10.0    10240   2015-07-29  
Windows 10, Version 1511    10.0    10586   2015-11-10  
Windows 10, Version 1607    10.0    14393   2016-08-02  
Windows 10, Version 1703    10.0    15063   2017-04-05  
Windows 10, Version 1709    10.0    16299   2017-10-17  
Windows 10, Version 1803    10.0    17134   2018-04-30  
Windows 10, Version 1809    10.0    17763   2018-11-13  
Windows 10, Version 1903    10.0    18362   2019-05-21