When you send a text message from your phone, you type the text, and that's pretty much all there is to it in terms of the content's appearance. You're limited to choosing whether you want to use capital letters. If you want more control over your message, you can switch to sending an email message, which might enable you to select bold, italic, or underlined text.
If you want to send highly formatted content, text with headings, font changes, text color, bulleted lists, and so forth, you might use word processing software, such as Word, Pages, or LibreOffice.
This class of software creates what is called rich text, that is, text rich in styling, and the generic term for software capable of creating a document containing rich text is rich text editor.
Drupal 8 includes a rich text editor called CKEditor. It is a highly configurable editor with a composition area, as shown in the following screenshot, that resembles standalone word processors with formatting buttons...