What is the meaning of the subscript x at the bottom of the summation sign in the definition of the mean of the discrete random variable?

I am used to seeing the summation sign with the 'starting' point for summation at the bottom and the 'ending' at the top. I guess x here is 'x belongs to whatever interval is relevant for the distribution you are dealing with'? (eg binomial from x=1,2, ...n etc?)