“Plain text” is just how it sounds– plain, simple text, without any enhancements. It is text without additional fonts, designs, or colors added. With plain text email, you won't see additional graphics or embedded multimedia. Even links aren't embedded in plain text email.
Change the format of all new messages\n\n On the File tab, choose Options > Mail. Under Compose messages, in the Compose messages in this format list, click HTML, Plain Text, or Rich Text.