Implied odds is the relationship between the size of the current pot and the pot you're expected to win. Because sometimes the pot doesn't lay the correct odds, even when you decide to play. Because you're expecting to get more action and win more when you hit your hand.
Implied odds changes things. For example, in Limit Hold'em your opponent bets $20 into an $80 pot and your call gives you pot odds of 5-1 (you're risking $20 to win $100). But, if you expect your opponent to call a bet or raise on the river if you make your hand, your implied odds are 6-1 or 7-1.