>5:5 means 'loud and clear'.
Actually the first "5" is "readability" and the second is signal strength. For CW (morse code) there's a third number for "Tone" or quality of the signal. The third parameter usually isn't included in voice communications.
Readability varies from 1 to 5 and both signal strength and tone vary from 1 to 9. So "5:5" or "5 by 5" means that it's perfectly readable with average signal strength. "5:9" would be perfectly readable with very strong signal strength. Go watch "Aliens". You'll hear "5 by 5" used by the pilot.
https://en.wikipedia.org/wiki/R-S-T_system