Respective rating used to report ballpark signal strength and clarity of a transmission. So a 5 by 1 means a strong signal is present, but what is saying cannot be understood. 1 by 5 means you can understand them, but a signal is barely coming in. 5 by 5 is ideal for both counts.
Can also be used nowadays to mean one is doing well.
This is an old aeronautical radio term. Radio signal strength and clarity were measured on a scale of 1 to 5. Today the term "Loud and Clear" is preferred to "5 by 5".
In slang use "5 by 5" means things are well, good, or as expected.
Also refers to level of orientation in a neurological/psychiatric exam. Five by five means that you are oriented to (your)name, place (where you are), time, day/date, and year/current President. A way of saying that you're alright, everything's cool.