When you whine, you complain in a pitiful, annoying way. Your younger sister will whine if your piece of cake is bigger than hers.
Whine is both a noun and a verb. The verb means "to complain bitterly." Kids tend to cry or even yell when they whine, though most adults settle for constant griping and an unwillingness to let it go already. "Poor me" and "It isn't fair" — these are common whines. Whine can describe a high-pitched, screeching noise, like the squeak made by an old, broken machine.