Short take:
Famous signal received on 11:16 p.m.EDT August 15, 1977 by the Big Ear radio telescope as part of the SETI program, and considered the closest "near miss" in SETI history. After 20 years, no confirmation or explanation for this signal has been found, making it more or less completely inconclusive as evidence for an artificial signal.

Longer explanation:
Poring over computer printouts, Dr. Jerry R. Ehman noticed the string 6EQUJ5 in the second channel. He circled this pattern and wrote the word "Wow!" that has since been so ubiquitous in SETI literature. Each character represented the average strength of the radio signal over a 12-second period. Thus the entire Wow! signal is basically a single, low-resolution, 72-second narrow-band burst of radio waves emerging from somewhere in the Sagittarius region of the sky. (The actual time resolution of the telescope was ten times greater, but this was averaged to save printout space.) The whole of extant Wow!, by Dr. Ehman's admission, is six data points. While the signal's source has not been pinpointed, that area of the sky was subsequently searched in detail to no avail.

There are two possibilities--the null hypothesis is that it is a fluke of one kind or another, and the alternative being that it was an artificial, extraterrestial signal that for some reason abruptly stopped transmitting or changed direction. With only six questionable data points to work with, there is no choice but to tentatively accept the null hypothesis. Extraordinary claims demand extraordinary evidence.