Extortion means forcing someone into giving you something through threats. A bully who tells kids he'll beat them up if they don't give him their lunch money is guilty of extortion.
Because extortion is a crime — mobsters are often charged with it — the word has a strict, legal meaning. It's the act of using threats or force to obtain something from someone. But the word is used in a general context as well. As you sell $5 boxes of M&Ms door-to-door to pay for your band trip to Florida, cranky neighbors may complain, "At this price, it's extortion!" — meaning they feel you're forcing them to give you their money.