Blackmail is the act of threatening to do something perceived as unpleasant unless the threatened party fulfils certain demands. A common blackmail threat is to reveal information, usually of an embarrassing or socially damaging nature. Another is to expose embarrassing photographs (possibly showing the subject naked), video or a sound tape of the threatened person (or someone they care about).
In the context of BDSM, blackmail is a common fantasy amongst submissives, who may get excitement from being forced to do something (especially sexual) to avoid something 'worse' (such as having their naked photographs delivered to their workplace). Pro dommes often feature blackmail among the services they offer.