It just means that states can't declare that debts can be paid in seashells or ReaganDollars, though they can, if they want, declare that debts can be paid in gold or silver.
The concept of legal tender is that creditors must accept it as payment for any debt. If they say I owe them $500, while they can accept payment in whatever fashion makes them happy, they have to accept cash and can't demand it in chickens instead. The bit in the constitution prevents individual states from declaring things other than gold and silver to be legal tender.
