Definition of implied odds
The implied odds is the money you think you will make on the upcoming streets if you hit your draw. They are difficult to calculate since it is a matter of estimating the amount of money our opponent will be willing to pay on the turn or river if you hit your draw. Nevertheless, they must be taken into account. When calculating the cotes in the pot and arriving at a tight (or even slightly losing) decision, you can justify a call by talking about the implied odds.
An example of a implied odds
You're at the turn with a triple (99 on 9-5-4-J). You are convinced that your opponent has a flush. But he's the stubborn type and you know he won't put down his flush, no matter what river you bet or raise on the river. He bets half the pot on the turn. Normally you would think that you're going to hit your full hand 1 time out of 5 on the river and you can't mathematically justify a call on the turn of a 50% pot bet. But if we know that when we touch our full hand, our opponent will be able to pay us big, we will say that we have the implied odds to call the bet. Generally, the less experienced our opponent is, the more good implied odds we have.