In the memo, FBI agent Guy Hottel records what an "investigator for the Air Forces" (whose name is blacked out) told him about what is popularly called "the Roswell incident". Hottel writes:
Three so-called flying saucers had been recovered in New Mexico . . . they were described as being circular in shape with raised centres, approximately 50 feet in diameter . . . Each one was occupied by three bodies of human shape but only 3 feet tall, dressed in a metallic cloth of a very fine texture. Each body was bandaged in a manner similar to the blackout suits used by speed flyers and test pilots.
The informant speculates that the saucers were found in New Mexico because the US government had a "high powered radar set up" there, and "it is believed that the radar interferes with the controlling mechanism of the saucers".
This memo seems to confirm what many believed happened at Roswell, which includes flying saucers crashing and alien autopsies (at left, you can see a 1947 FBI memo about Roswell). But it's important to remember that even if the memo is authentic, it is simply the record of what one informant said to another. We don't know who this Air Force investigator is, nor do we know whether he's reporting what other people told him, or if he witnessed the saucers himself. He could be simply reporting a rumour. (For a smart skeptic's view, see Steven Novella's comments on the memo.)
Over at the International Business Times, Jesse Emspack calls the memo part of a 50-year-old hoax, and explains how exactly this authentic FBI document is just the last part of a long paper trail created by a fake UFO sighting. He says the memo has been around for a while, but only this week became accessible via the FBI vault site.
You can see the memo at right (click to embiggen), or download the full memo from the FBI vault website.
Thanks for the tips, Hugh D'Andrade!