How to build a constraint with summation?

If Q is a column of 79 values, then why it is indexed this way sum(Q[a,w]*x[a,t] for a in A) inside the constraint (where A is 1:79 and w is 5)? If Q is a single column then it can be indexed with two indexes only if one of them is always 1, like pointed by odow in his last reply.