5x5 (five-by-five) is a military radio communications expression that means 'loud and clear' also ‘lima charlie’ in the military phonetic alphabet. The 1st five represents the rating of signal strength/volume. The 2nd five is a rating of signal clarity. Signal is rated from lowest as 1 to best as 5. So 5x5 is a good, clear, best possible signal. In modern vernacular it also means 'all good'.