Often when writing mel scripts, we may have a number of settings for our tools and commands. One example would be creating a polygon cube. The default settings for this are stored so that if you close Maya, the next time you open Maya and create a cube; it will use the previous settings.
optionVar is basically a little mel command that allows us to store some tool settings for the next time Maya opens. First off though, if you execute the command
optionVar -list;
Maya will list all current user options it is storing.