I was very amused the other day by one of my client's use of this phrase. It comes from the early days of PCs (the 1980's) when it was a breakthrough to get things to print out the way they looked on screen, and WYSIWYG was a sales feature. It then slipped into common usage (at least among IT people) to mean simply that someone is very straightforward - ie they are truly just who they seem to be.
In our session we had been exploring reciprocity and perceptions (vs reality) in this case in relation to trust. To cut a long story short, through some 2-chair work we'd seen that if you go into a meeting treating the other person as a threat (so not to be trusted) you will naturally be suspicious of their motives and cautious with them. Reciprocity of emotional response means that they will immediately read this caution in you (at least unconsciously, sometimes consciously) causing them to become similarly suspicious and cautious. So both sets of defences are up, all comments or actions will be interpreted as potentially hostile and each side will actually experience the other as not to be trusted. Perception becomes reality!
The helpful part of these ideas is opening up the possibility that just because you experience another person as threatening it doesn't always mean they really are - it may be your perception that is creating this reality in your interactions. By re-framing the situation so that you perceive them differently, you may actually experience a different them.
What I loved is that my client (an IT person of roughly my vintage) summed this up as "what you see is what you get" - a deliberate reversal of the normal meaning of this phrase which was (I thought) very clever and quite witty.
