It means that Canadian teams can't pay a player more to make up the difference from having to pay taxes.

To fit within the cap, they have to pay the player (for example) the same $5 million that Florida team offers, but because the player doesn't pay taxes in Florida, the $5 million is worth more. So without a salary cap, the Canadian team could offer $6 million to make up the difference, but can't because they are limited by the cap.