What You See Is What You Get, commonly abbreviated to the acronym WYSIWYG is a basis for the entire Games Workshop line of models. It simply states that whatever kind of wargear or weapons that the model is going to have equipped going into battle MUST be shown on the model. For example, a Space Marine usually is armed with a bolter as part of their standard equipment. In order for everyone who looks at the space marine to know that he has a bolter and not another type of weapon (Flamer, Missile Launcher etc.) a bolter must be physically attached to the model.
Some types of wargear also must be displayed on the model when appropriate. These wargear items are displayed when Games Workshop models have provided for said wargear. Otherwise, it is up to the player to include these wargear items as imagination permits.
It should be noted that this term's officiality has come and gone over the years. Tournaments normally give a standard of models that can be used in play, and many have had very lenient policies. Sometimes, one type of miniature could be allowed as a proxy for another. However, often the rules are written to discourage such actions in order to prevent abuse.