In WordPress, a text editor is a window that opens on your screen that allows you to create, modify, or delete information in a file, whether the file is a post, a page, or a template file.
In general, text editors are very basic programs in a visual interface format that allow you to add, modify, or delete text. For instance, Notepad and TextEditor are two popular text editor programs that come built-in with the Windows and Macintosh operating systems, respectively. More powerful text editors include apps like Microsoft Word or Google Docs.
In WordPress specifically, the text editor is most often used when creating a new post or page.
The WordPress text editor can be used in one of two modes: Visual and Text. With Visual, what you see in the text editor is what will be displayed on the screen. With Text, however, you’ll need to input the formatting information (such as style, font, bold, italic, etc.) manually using HTML. The only formatting that Text will do automatically is adding a line break when you start a new line of text (by pressing Enter twice).
At the top of a WordPress text editor, you’ll see a toolbar with clickable icons that activate commonly used formatting commands. Keep in mind that your choice of either Visual or Text in the text editor will be “remembered” by WordPress and automatically selected the next time you create or modify a post or a page.
Note: There are a number of plugins available to add more functionality to the WordPress text editor.
Further Reading: The Best Text Editors