Toil is a Numeric Loaded Keyword that modifies the cost of a card.
From the Comprehensive Rules 4.0:
When you play a card that has the toil keyword, you may reduce its twilight cost by the number specified. You do this by exerting one of your characters of the same culture as that card.
You may exert multiple characters of the same culture. Each different character you exert reduces the twilight cost by the number specified. You may exert a given character only once when playing a toil card.