If you don't roll it over into another eligible retirement fund, you could incur penalties if you under 55 and no longer with that company. (sapling.com)
You will be taxed on a distribution from the Plan if you do not roll it over. (franklintempleton.com)
When one expires, you just roll it over into a new CD until you're ready to pull the money back out and use it. (mybanktracker.com)