What does this mean and what's the answer

It means to find the answer. The answer is 17. 
2 x { 6 + [ 12 ÷ ( 3 + 1 )]} - 1.
You first need to do 3+1=4.
Then take the 4 from 3+1 and do: 12÷4=3.
Then take the 3 from 12÷4 and do: 3+6=9.
Then take the 9 from 3+6 and do: 9x2=18.
Finally take the 18 from 9x2 and do: 18-1=17
Hope this helps! :D