If you look at this image: https://markdownpad.com/img/markdownpad2.png
The "Syntax Highlighting" is the color and styling used to identify specific Markdown formating in the Editor pane (the raw Markdown text). If you look at the Headings (
##) for example, you will see that they are red, bold text is bolded, and code blocks are green. If "Markdown Syntax Highlighting" is enabled, those colors and styles will be displayed. If it is disabled, only plain text will be displayed.
Does this make the translation more clear for you?