5:5 means five by five.
Signals comm term
Meaning:
Signal is loud and clear
Five by five is the best of 25 possible subjective responses used to describe the quality of communications, specifically the signal-to-noise ratio. As receiving stations move away from an analog radio transmitting site, the signal strength decreases gradually, causing the relative noise level to increase.
Five by five - Wikipedia, the free encyclopedia
this has been explained many times