http://lmgtfy.com/?q=what+does+I+hear+you+five+by+five+mean#seen
Thomas Gibson has been fired from Criminal minds now I won't watch CM, No Hotch no watch

Loud and Clear is all I used, or ever heard used.
Push the button, Max

Interesting to see Island's example of 599 in his explanation.
On my captions, when the pilot says "5 by 5", the caption read "555".
You Fill Me with Inertia.

"RST" is not an acronym.
Sure is
https://en.wikipedia.org/wiki/RST_code

reported in RST(s), acronym[sic] for Readability, Strength and Tone.
"RST" is
not
an acronym.
Everyone may have an opinion but very few seem to have an informed one.

In the world of radio communications, voice and CW (Continuous Wave aka Morse Code) signals are measured and reported in RST(s), acronym for Readability, Strength and Tone.
5 is the highest signal level for Readability, 9 is the highest signal level for strength and 9 is the highest level of Tone.
Readability and Tone are based on actual hearing observations while Strength is based on instrument or meter readings.
5x9x9 or 599 is the best signal report one can give.
Of late, Morse Code use has declined steadily but SSB or Single-Side-Band voice communication over-the-air is still very popular especially among Amateur Radio Operators otherwise known as HAMS.
In voice communication, the third number is dropped, thus 5x9 or 59 is a perfect report. 3x1 or anything lower is considered very poor signal for voice communications.
If one owns a Radio capable of receiving "Short-Wave" broadcast, scan up and down the Bands and tune in to a frequency where radio operators are having a QSO (Conversation). One may hear these terms used very often in these exchanges.

5 by 5 is loud and clear. I believe 5 by zero is pronounced
ssshhhcracklewheeeesssh

http://en.wikipedia.org/wiki/5x5

I believe in that case it would be 0 !

One by one.

what's the lowest?
1 by 1 or 0 by 0?
And if it is the later, how is the 0 pronounced?

Actually, it's a report of both signal strength and clarity. 5x5 means strong signal, good voice clarity. 5x2 might mean the signal is good, but the voice is garbled or covered up by background noise or a bad mic. 2x4 would mean that the signal is weak, but the voice is still understandable. Etc.
I'm just a patsy!

This message has been deleted.

IIRC
five by five - perfect vocal reception.
three by three - communication intermittently breaking-up or partially diminished
one by one - essentially getting only unintelligible or "no voice" audio.
On November 6, 2012God blessed America