The WYSIWYG (What You See Is What You Get) text editor, also known as a Rich Text editor, is where you write your text. It's also one of the places in your Drupal site where you can embed media objects and links to present them to your audience.

A rich text editor on the web is kind of similar to a word processing application such as Microsoft Word or Open Office Writer.  It has a UI (User Interface) with buttons on it and it performs some similar functions, such as the ability to add header elements, hyperlinks, and create nested lists.

It should be remembered, however, that most web-based text editors are not high level word processors and the environment you're working in, which is on the web versus on your local computer, adds in some additional workflow variation that you may have to get accustomed to.