A term used in Gondor in the mid-late Third Age to denote those of the Dúnedain of Gondor who fled to Umbar after their defeat in the Kin-strife (T.A. 1448), largely descended from Castamir the Usurper and his followers; and their racial and cultural descendants among the other Umbareans and Black Númenóreans of Umbar, beginning a long enmity against the land of their ancestors. By the time of the War of the Ring, these peoples had largely melded into one people or culture, mostly Haradrim in nature, who inherited undiminished their hatred for Gondor.
A corsair is simply a privateer or pirate; in this case the term indicates their primary endeavors: to harass Gondor and it's shipping and maritime activities in any and all ways possible; and occasionally to launch large-scale raids against it's coastal areas, for booty, slaves, and/or fun. Though needing little to no encouragement from outside, in this they were no doubt often in league with the Dark Lord.