WYSIWYG (pronounced "wiz-ee-wig") stands for "What You See Is What You Get". Typically this refers to a graphical user interface which shows you the final output, instead of the underlying code.
A classic example is a web content management system which shows you what your page looks like with bold text and images etc in place... instead of showing you the underlying code which looks like:
<b>bold text</b> .... align:left ..... <img src="..."> and so on. WYSIWYG editors make it possible for anyone to create content without necessarily having to understand the underlying syntax and code.