It means the manufacturer has already overclocked it by a certain amount. Most cards can be overclocked above and beyond this level. Overclocked cards by the manufacturer is safe to use in your computer and won't generate any additional heat than what is already stated on the box.
Basically manufacturers do this to charge you an extra 10-40 dollars since they assume most are going to do it anyway.

You don't have to get an overclocked one but it would be slower. In the end it is still safe to get these cards and do no additional overclocking.