Visual editor is the default WordPress editor to write content either in a post or a page. You can create and edit your text in rich format just like Microsoft Word. Moreover it is a WYSIWYG (What You See Is What You Get) editor which means the exact text formatting will be appeared on the front end.
With the help of toolbar, you can create headings, make your text bold, italic, create lists and much more.
Using Add Media button, you can add images, audio or even videos to a post.
If the toolbar in WordPress post is not appearing, make sure you are in Visual tab on the far right corner.
If you click the Text mode on the far right hand side, WordPress visual editor will be switched to text editor. See what is text editor in WordPress for more details.