Sometimes you want to use text that was written before. For example a text file on your computer. Text editing applications like Word or Open Office do not only save text in a file, but also styling of text. (font weight, color or headers for example) These styles will not be recognized by a web-page, and could result in strange results on screen.
It is not too complicated to filter these styles from the text. Just paste your text in Notepad (Windows application) and re-copy the text from there before pasting in the CMS editor. That way you are sure you are saving "plain text" in the CMS.
On Macintosh computers: First paste your text in Text editor. Choose form the menu: Format > Make Plain Text (or Shift+Command+T). After that, copy your text again, and paste into the CMS editor.