Programming language source code is written in plain text. You can open up a simple text editor that comes with your operating system like Notepad on the PC or TextEdit on the Mac, and they work just fine to write any programming language. There's nothing magical about source code itself. It's just text. And that's plain text, not rich text.
Programming language source code does not need to be bolded or underlined or italicized or justified. So say if you're using a program like TextEdit on a Mac that can work in the either rich text or plain text modes, you want Plain Text. Now programs don't actually have to be much more than this. In fact these would all be considered very simple but technically complete programs in these languages. Just one statement, one instruction that outputs the words Hello, world on to the screen. Now Hello, world is the classic example of this simplest program imaginable in any language.
You see many languages share a common history and they are often more alike than they are different. Now just because this statement is the same in these languages does not mean these languages are identical. Far from it, but there are often significant similarities between languages. Now, some languages do need a little more than one single statement to be considered a full program.
Now if you do any web development you might find that your chosen web development application is also a usable programmer's text editor. And then we have Integrated Development Environments or IDEs. These are large programs and include a good programmer's text editor, but usually add on a whole bunch of features for professional development. Examples of IDEs would be Apple's Xcode on the Mac or Microsoft's Visual Studio or Eclipse, which is a cross-platform.