What is Rich Text in the Computer?
Rich text is text that can be bold, italic, underlined, colored, include images and videos between paragraphs, among countless other features. The term contrasts with plain text, which is a very simple text format that doesn't even have the concept of paragraphs.
Rich text isn't a single format, but a term that refers to any format that provides rich text features, meaning that two different applications that support rich text don't necessarily use the exact same format, and even things like copying and pasting from one application to another may not work.
Documents created in rich text editors such as Microsoft Office Word and LibreOffice Writer are rich text documents. PDF's are rich text documents. And web pages are rich text documents as well.