It has a similar meaning to research. What it means is that you can reproduce (compile in most cases) from source code the same bit-for-bit identical binary independently. While this might sound like something that should be trivial to do, it turns out to be far from trivial (timestamps and other environment information leaks into binaries all the time).

There's a website that describes this project in much more detail as well as how they worked around the various problems they found. https://reproducible-builds.org/