Datakeys on Gridview

I am stuck in the following line of code: 
int pollID = Convert.ToInt32(gvwPolls.DataKeys[Convert.ToInt32(e.CommandArgument)] [0])

Just one question:

What is the purpose of "[0]" in the line above? If I remove it, the code compiles OK but I get an error message at run time... Any Ideas?
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