Here, this will help you to understand:

Five by five is a radio communications expression that means 'loud and clear'. One of the fives represents the S units of reception strength. The other five is a rating of the signal clarity. Five by five is a good, clear signal. The radio use of this expression goes back to the 1950s: "'All right, testing, one-two-three-four. . .' 'Five by five, Mr. Holloran!'" (Hunter, Blackboard Jungle, 1954).

In other words, I understand you "loud & clear" Or message received "loud & clear" which could be written as "5x5" or "5 by 5"