Double rails on a bodyboard refers the number of layers of rails laminated to the core. Most boards over $100 will have double rails as it improve's the overall strength of the board & provide's more grip for the outside hand down the rail. The inside rail is usually referred to as the "pinline" & the outside rail as the "rail"
Double rails usually come in ratio's 60/40, 55/45 & 50/50. This refers to the ratio of the bottom rail as opposed to the top chine. The bigget the bottom rail the harder it is to catch a rail but also harder to get an edge
60/40 is very forgiving to beginner's etc
55/45 is a good balance.
50/50 is very easy to catch edges but turns very fast & responsively.