Reproducible builds are a set of software development practices that create “a verifiable path from human readable source code to the binary code used by computers”. In other words, if you don’t change the source code, the compilation result should always be exactly the same.
Explained more simply in the case of Symfony: if you build the container and warm up the cache of the same unchanged application multiple times, the result should always be the same.
The post talks about the idea of "reproducible builds" and how they should be "completely deterministic" where the end result is always the same (no random data, no auto-generates date/times). A few changes were required to the framework to ensure these builds were possible. The post lists out these updates and links to the bug reports for each.