In voice procedure (the techniques used to facilitate spoken communication over two-way radios) a station may request a report on the quality and strength of signal they are broadcasting. In the military of the NATO countries, and other organizations, 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 that has excellent strength and perfect clarity -- the most understandable signal possible.

Five by five by extension has come to mean "I understand you perfectly" in situations other than radio communication, the way Loud and Clear entered slang, post-WW2.

It is sometimes used as slang term to mean "everything is good". The character "Faith" from the "Buffy The Vampire" TV series used it this way.

Sometimes also used in athletic training.

A euphemism for “kill me” now that composting oneself after one’s death has become popular.

Actually, it is a radio term but it means Loud and Clear, not I hear you and you hear me.

Used in "Buffy the Vampire Slayer" by the character Faith, the second slayer to mean all is good

It's a radio term. It means I hear you with adequate signal strength and adequate clarity. The scale of each of these was 1 through 10, with ten the strongest. Technically, a 10x10 would be the best. But anything down to 5x5, like a 6x8 or a 5x7, would be understandable and adequate. It has taken on many new meanings, but all seem to be positive, or OK. Even though what it's based on would mean 5x5 was mediocre at best.

On track, on course, going where you want to; it also refers to the central point in a targeting scope - five over and five up.

If you are "all square" with someone (as opposed to being just a square), or you are cool with that person, then you are 5 x 5.