Newbie here...when making a tiered cake, I know that each tier should be on a cake board, but my question is: should the cake board(s) be covered (ex. foil)? Or can you just put the cake directly on a cardboard cake board?

Lots of answers to that question.   Personally on the few tiered cakes I have done I have not covered the board.  I didn't want to have to worry about little slivers of saran wrap or press and seal or foil in pieces of cake in case the covering was cut.  But others cover in all of those things.  Someone on here seals their cardboard in a soy wax, some only buy waxed cardboards.  Some use foam core instead of cardboard. 

If you have bought the "bakers circles" that are corrugated and white on one side, you do not need foil.  That white side is greaseproof treated like baking parchment. That white side goes toward the cake.

I would suggest to make sure that bakers circles and other paper goods that touch the cake are always bought from a source that keeps them dust free.  Some shops buy them in bulk--they should be repackaged into plastic bags to remain sanitary.