By binary formats, they mean files that are intended to be interpreted or executed by a computer.

For example, a program (my-app.exe) is a binary file. You use it by double clicking it, and the computer runs the program. If you tried to read the file by opening it in a text editor, you would only see gibberish and random characters.

Another example, an image (my-photo.jpg) is a binary file. It’s not a program, but you can view the image with a program like Paint. But just like a program, if you opened an image in a text editor, you would still see gibberish. That’s because the gibberish is interpreted by Paint so that you can view the image.

But source code (my-website.html) is not a binary file, it’s just a text file with fancy formatting and special rules on how to write it. You can open an HTML file in your text editor, and as long as you know the rules you can learn to read it.