It's a slang expression for a girl who looks over the age of consent, but is in fact under age. The problem is particulalry acute in the U.S. where statutory rape (the offence of intercourse with a girl under 18) is rigourously enforced with a jail sentence. Various songs have been written on the subject - Nils Lofgren's 'Jailbait' advises "Well her friends call her Jailbait, youd have to agree, her age is just fourteen, but she looks twenty-three ..." which says it all really.