When you eavesdrop, you secretly listen in on someone's conversation. A little girl might eavesdrop on her parents in an attempt to find out what she's getting for her birthday.
Whenever you deliberately overhear a phone conversation, or two people having a quiet argument, you eavesdrop. You might eavesdrop on a funny exchange between two friends at a cafe, or eavesdrop on an important conversation between your boss and one of your coworkers. The verb eavesdrop comes from the old-fashioned noun eavesdropper, "one who listens from under the eaves," from the even more old-fashioned eavesdrop, "ground where rainwater drips off the roof."