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