It means almost nothing... unless you get sick or run over by a car. My family skated by for years without health insurance. We even had two babies while uninsured. We lucked out and came out ahead. We saved far more by not paying insurance premiums than we paid on checkups, vaccinations and even the delivery of two babies. If my wife had to have a c-section or there were other complications things would have been quite different.
Edit: Also, if you just lost your insurance and you want continued coverage, look into COBRA coverage.
Wow, so it is really possible to not have an insurance coverage until you get old. I know without insurance is financially risky but then seeing this post makes me think that I can still live without insurance. The amount of insurance we avail these days is really something these days so I think if you're not an accident prone magnet, you can still live without having to pay for insurance.