In World War II, strange glowing balls were observed in the air and photographed by Allied pilots. These unexplained phenomena came to be called Foo Fighters (the rock band came much later).
Some who speculate on the nature of extraterrestrial life see Foo Fighters as both UFOs (unidentified flying objects) and ETs (extraterrestrials). In other words, Foo Fighters would be energy beings like we see in Star Trek and other science fiction dramas.
There are, however, other more down to Earth theories about their existence, outlined here: http://en.wikipedia.org/wiki/Foo_fighter