Plain text format means text contains no formatting such as bold/italics, coloured fonts, different font styles, etc. Rich text format allows text to be formatted with colours applied, etc. In a CMS if you have plain text only, using HTML tags would not format the text. For example, if you entered <b>Bold</b> that’s exactly what would be displayed on your site. But if you’re using a Rich Text editor, it would pick up the HTML tags and Bold would be displayed on your site.