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.
In two way radio, a call would be, "how do you receive me?", to which if the signal was acceptable, the answer would be, "I read you 5x5."