Rabbit hunting means showing the remaining community cards after a hand is finished and the winner is known.
For example, if you fold a flush draw on the turn after your opponent moves all in, you can ask the dealer to show what would have hit on the river to see if you would have made your flush. Seeing such a card is known as rabbit hunting in poker.
Usually, it is not allowed to see additional cards when the hand is over since it slows down the game and can even cause negative emotions for the players involved. That said, the dealer might agree to reveal what would have come next in a specific situation or big pot, but you should never abuse it.
These days, you can even use this function when playing online since sites like GGpoker readily offers you to see the next card even when you fold with a click of a button. In such a setup, there is no harm in seeing the card, so if you want to have some fun and speculate how the hand could have ended, go ahead and rabbit hunt.