Implied Odds in Poker is an extension of Pot Odds. Implied Pot Odds are calculated the same way you calculate Pot Odds but with Implied Pot Odds you consider future betting rounds. Implied Odds are figured in situations where you will likely fold on the next round of betting if you miss your draw, but if you make the hand your drawing to then you will gain additional bets from the player(s) who call.
You can add these expected additional bets to the current pot and this gives you the implied pot. When you use the Implied Pot when calculating your Pot Odds decision, you are calculating the Implied Pot Odds.
Often times a call will appear to have a negative expectation when looking only at the current pot odds, but when you consider the additional bets on the next betting round that you will gain if you make your hand then the call may become a positive expectation play.
Texas Holdem poker example,, you are facing a $20 call into a $100 pot during the betting round before the river card is dealt, a 1:5 (20%) decision. You have an open ended straight draw giving you 8 outs out of the 46 unseen cards. 8:46 (17.4%). Looking at your pot odds, this call would have a negative expectation. However, if you add the additional $20 bet you will earn on the river if you make your straight draw, then your implied pot will be $120. So the $20 call you face actually wins $120, making it 1:6 (16.7%) decision. Since the odds of making your hand is 17.4%, which is greater then the implied pot odds of 16.7%, making the call here has a positive expectation.