To kvetch is to grumble and complain at length. Kids who kvetch for days about never having gone to Disney World are unlikely to convince their parents to take them.
When you kvetch, you whine or grumble or bellyache about something. It's an informal way to describe a very annoying activity, and you can also call the complaint itself a kvetch, as well as the person doing the complaining. In fact, this it the original Yiddish meaning, from kvetshn, "squeeze" or "press," which has a German root, quetsche, or "crusher."