In the military of the NATO countries, and other organisations, the signal quality is reported on two scales; the first is for signal strength, and the second for signal clarity. Both these scales range from one to five, where one is the worst and five is the best. The listening station reports these numbers separated with the word 'by'.
Five by five therefore means a signal which has excellent strength and perfect clarity - the most understandable signal possible.

The term "five by five" was popularized by the character Faith (played by Eliza Dushku) who was a "Slayer" in the television show Buffy the Vampire Slayer and its spinoff Angel.