It is one of the two post editor views currently included in WordPress. The other one is the text editor. The text editor displays the content of your post as unstyled HTML code, which gives you more precise control over the formatting but is less intuitive – especially if you’re not comfortable with writing HTML. You can switch between the two different editors by clicking the tabs in top right corner of the editing view.
The biggest benefit of the visual editor is that you can get a good sense of the end result without clicking the “Preview Changes” button once every five minutes. Embedded videos are loaded in the editor itself, galleries are displayed with previews of the images, and you get a better sense of the formatting of the text in the post. This is what’s called a WYSIWYG – What You See Is What You Get – editor.
The visual editor also contains a good selection of buttons and keyboard shortcuts that you can use to format the text. If you copy text from another source, like a Word document, the visual editor will try to match the styling of the copied contents when it displays it in the editor. Depending on where you copied it from, this can sometimes misfire and turn your content into a jumbled mess when you preview the post. If that happens, you can remove the pasted content, switch to the text mode in the top right, paste it, and switch back.
Themes can include editor styles to make the visual editor look the same as how the content is presented on the site. If the theme does not include its own editor styles, the default WordPress editor styles are used.