The details and specific target of the program have yet to be made public. The New York Times, citing unnamed officials, says that the CIA had plotted since 2001 to find and kill al-Qaeda leaders abroad. Two former CIA officials tell TIME there's another, somewhat less dramatic, possibility: a plan to conduct domestic surveillance. Spying on Americans is outside the CIA's purview and would be highly controversial — good enough reason for Cheney to want it kept under wraps.
Less dramatic perhaps, but far more troubling as such a program is more than "outside the CIA's purveiw"; it's blatantly illegal. And it would explain why Cheney would want so badly to keep it a secret from Congress.