This is military slang meaning “all good.”
It comes from how they used to measure radio signal (1) strength and (2) clarity on a scale of 1 to 5. So, a strong signal that was distorted would be “five-by-one” and a weak, clear signal would be “one-by-five.” A strong clear signal would be “five-by-five.”
“I hear you, five-by-five” came to mean “loud and clear” (again, strength and clarity) and this morphed into a general acknowledgment of understanding and security.
Why I Looked It Up
I was reading a military novel, and it came up often. A character checked in with a bunch of sniper posts, and they all responded that “everything is five-by-five” or some variation.
I also remember it from the legendary Mall Ninja story.
Basically, the situation is that we get the call, we lock up the situation, put everything five by five, and cordon the area until the local authorities arrive.