It's a term for binaries (usually ELF) being byte-to-byte equal in two
different runs of a compiler. This way you can build a binary package from
source package and if its content is the same, you know what source code was
used to build the package, and then you can e.g. inspect the code for
backdoors or build debug symbols without planning for that beforehand.
https://en.wikipedia.org/wiki/Reproducible_build