Thus far, Microsoft's Windows 10 codenames have been pretty straightforward. Originally, it was called Threshold, and that ended up being broken into two parts to meet a July, 2015 release, with Threshold 2 being released in November as version 1511. The next wave was codenamed Redstone, and that's the name we've known ever since with Redstone 2, Redstone 3, Redstone 4, and the upcoming Redstone 5.
But after Redstone 5 is released this fall, Windows 10 codenames will have a new format, according to a report from Windows Central. Moving forward, we'll see names like 19H1, 19H2, 20H1, and 20H2.
The codenames are actually pretty similar to the way that Windows 10 version numbers work. With version numbers, the first two digits are the year and the last two are the month, so version 1803 is finalized in March, 2018. Microsoft doesn't have a set date for when something will be done as soon as it starts working on a feature update though, so the month is replaced by H1 and H2, reflecting the first and second halves of the year.
It also leaves some flexibility. While the upcoming Windows 10 spring update is most certainly version 1803, the Xbox One update will be version 1804. While they have different version numbers and won't be done at the same time, they both run Windows 10, and they'll both be done in the first half.
This change only reflects internal codenames, and the build string that you see when a new build is installing on your PC. Once the update is finalized, it will still have a regular version number. According to the report, this also won't affect product names, like Anniversary Update and Creators Update. Unfortunately, Microsoft still hasn't released an official name for version 1803, which should be out within weeks, so it's entirely possible that the company is ditching those names anyway.
All you really need to know is that there won't be a Redstone 6. Redstone 5 will be finalized this fall, and we'll see Insider builds for 19H1, which will ship in the first half of 2019.