WYSIWYM means "What You See is What You Mean" and is enabled through the Show Blocks button  in the editor. When toggled, this allows the user to see the content in "blocks" with a visual representation of how content is marked up:

This also allows one to see if there is any extraneous markup causing issues, without having to dig through the Source view, if editing code is not your forte´.

More information on WYSISYM itself can be found at: http://en.wikipedia.org/wiki/WYSIWYM