WYSIWYG is an acronym for "What You See Is What You Get". It is a system that lets user to see how editing results appear in final produce version.
In games development, WYSIWYG is an interface that allows developer to view results that is very close to what we will se in the final version of the game.
There is also a variation of WYSIWYG term, WYSIAWYG ("What You See Is Almost What You Get") which means that only a part if editing results are shown. Most of game tools really using WYSIAWYG systems because of problems of full implementation of WYSIWYG.
WYSIAWYG centers around most noticed editing result properties (properties that are important to the developer). For instance, a light source radius and color are important properties because they sets the look of scene (the whole goal of making lighting), shadows and per-pixel rendering are less noticeable so can be sacrificed for the implementation simplicity of WYSIAWYG editor.