A judgement was awarded against me because I missed the court date. In the letter from the court it said that the costs of the action is to be "taxed" against me. I live in NC. What exactly does this mean?

It means that the final judgment against you will include the costs which the plaintiff had to spend in order to sue you. Costs typically include court filing fees, etc.

I am not sure what you mean by "the letter from the court". Is this really a letter or is it a paper filed in the court file, perhaps the judgment itself?

Typically costs are "taxed" by the attorney for the plaintiff presenting a cost bill to the clerk of court. If the clerk agrees that these items are properly awarded as costs, an appropriate document, "taxing" these costs, will be signed by the clerk and entered in the court file.