If you have done any kind of work with an html editor, than you have probably run into this term.
I actually first heard about it back in the mid 1980's when computers were still quite basic - mainly a storage system or a spreadsheet program that would make users cry today.
WYSIWYG is an acronym.
It stands for: What You See Is What You Get.
What it means is pretty much this:
When using an editor and choosing the font, colors, alignment, text types, etc - usually from a bar across the top of the editor, all of these choices will be shown as you type along.
Your finished product will look like it does as you prepare it.
No surprises - what you see is what you get.
Without this feature, you better know some basic html and the different names for fonts and colors so you can code the webpage and have it look correct.
WYSIWYG makes our life so much easier, and many computer users today don't know what it is like to not have this feature on an editor.