Comments

When people write software, the actual computer code that they write is typically accompanied by plain-English comments that describe what the code is supposed to do. The intent is to document what is being done in a meaningful way so that other programmers will be able to understand how a given section of software is supposed to work.

When a program is compiled (turned from programming code into actual "software", so to speak), these comments are filtered out, which means that typically programmers assume that the only people who will read them are other programmers.

But with open source software, where the actual raw programming code is available to everyone, comments become publicly available.