This term is applied to the compiled instructions contained within an executable file. Binary code is not human-readable and can only be understood by the computer’s processor when the program is run.
Source code, by contrast, is made up of the statements created by a programmer using a text editor. Source code is human-readable, for anyone who understands the conventions used by that programming language (‘C’, ‘C++’, etc.), but cannot be executed by a computer’s processor until it has been compiled.