It means your land may have mineral interests from long ago that were granted separately to a mining company (or some individual), though nobody has used the rights, but nothing prevents from being exercised in the future. The title company doesn't want to accept the risk.
Because of the serious impact an actual mining operation may have on a property, the title insurance company is saying its policy prevents it from covering you, as it is NOT a defect in title (merely a choice you get to make). If you accept the deed you accept the risk.
You might file an action to quiet title and have the ancient rights terminated, if it's that big a deal for you. Or you can track down the owners (descendants) and buy back the rights. Good luck with that one.