[R] a question about using the sensitivity package

[Jiang, Yueyang]

Hi,

I'm using the sensitivity package to calculate the sobol indices. The case is 
that I have 13 parameter and each one has 1000 samples and my model result is a 
vector having 1000 values (based on the 1000 sample simulation).
I just wonder how to use the sobol function, the pdf looks straightforward, but 
actually it is difficult to use. 

In the sobol function, it has X1 and X2, what are they? How can I organize my 
parameter sample matrix (1000 by 13) and model result (1000 by 1) in the sobol 
function? 

Could some one help me for this question?