WYSIWYG is an acronym for "What you see is what you get." from English (What You See Is What You Get).
WYSIWYG involves an interface that allows the user to see something very similar to the final result while the document is being created. In general, WYSIWYG requires the ability to directly manipulate the appearance of a document without the need for advanced knowledge.
When a web page is created or a document is written using a WYSIWYG editor (or word processor), the editor displays the page exactly (or almost exactly) as it will appear when published on the Internet or when printed (if we are talking about a document).
Most CMSs have built-in such editors.