I'm sorry for the snark in my comment, it intended to just be a funny joke due to the capitalization thing (in Go that's what separates public from private fields, which is a weirdness of the language that surprises people the first time they get to learn it)

As others said, while "open" does indeed mean "reachable" or "available" in this context of source code, it happens that "open source" is a well defined thing to allow not only access, but also modification, reuse, and distribution without limitations. So the "open" in "open source" has its meaning brought to the highest level of openness.