Dunno if anyone has answered this so I will.

The US intercepts all calls with any party outside of the United States. When those calls have an American citizen on the other end, the identity of the American is to be blacked out in any transcripts unless a warrant is obtained in a criminal investigation. "Unmasking" is the illegal activity of exposing of and spying on American citizens on the other end of such communications without cause. This how Democrats were spying on the communications of American citizens (in this instance).

It should come as no surprise that Samantha Power is the first person on that list. She's escaped a lot of attention as she was relatively obscure prior to getting a job with Obama but she and her husband are hard core communists, like Brennan.