To mooch is to take advantage of other people's generosity without giving anything in return. If you constantly mooch rides from your friend, she's going to get tired of agreeing to drive you around.
When you mooch food from your neighbors — showing up every night at dinnertime, or appearing uninvited at their family barbecues — you help yourself to free meals. A person in the habit of doing this is also called a mooch. The word's original definition, "pretend poverty," might stem from the Middle English word mucchen, "to be stingy," or literally, "to keep coins in one's nightcap."