It means your payout will be 2 in your 1 bet. Overall, you will get 3 since it’s 2(profit)+1bet.
The chance of winning those bet is 1/3 so they are paying x3 minus the house edge. Also that bet means you are betting that 1 of the numbers in that column will win.
For example, the first column has 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31 and 34. If I choose the first 2:1, that means if the boule falls under any of these numbers, I win if I get you right. I thought as much but I just want to be sure before I start selecting it.
That is good.
The probably of winning it is truly 1:3 and the odd for it should be 3x.