WYSIWYG is an acronym for 'What You See Is What You Get'. It's a type of HTML editor that lets you make web pages without requiring HTML coding. Instead you simply insert pictures, text, links, and colors. You've probably seen this kind of editor if you've ever had a blog or used WordPress or another online form with content and then an editing toolbar up top.
It's a good idea to always use the same web editor program in which a site was originally created, as using a very different program can potentially corrupt the original design. The HTML Editor that you can find in cPanel via the File Manager section utilizes a WYSIWYG editor.
Additionally, WordPress (which can be installed with Softaculous in cPanel) also has the ability to add posts and pages using WYSIWYG. There are also many third-party HTML Editors available that use WYSIWYG.