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.