One of Bitcoin’s most important security features, a crucial property that makes the system trustless, is its open source nature. Because Bitcoin’s code is open source, anyone able to read code can check for themselves whether it does what it’s supposed to do.
While anyone able to read C++ can check and verify Bitcoin Core’s open source code, they still need to trust that the compiled and published binaries reflect this same code.
Gitian is an open source software program that offers what is known as a “build environment.” A build environment is perhaps best described as a “computer within a computer,” but with a specific purpose: a virtual space to compile binaries, completely free of any variables. “Gitian Building,” the process of compiling binaries in Gitian, ensures that whatever computer is used, the binaries turn out exactly the same.
This, in turn, enables different developers to reliably cross check their binaries against each other. And, of course, it allows anyone else to verify whether the binaries match the source code as well.