Sigma is a shorthand notation that means the sum of a number of terms having a common form. It looks like an M sideways. From the way i understand it, it represents sequences like 2, 4, 6, 8, 10, 12, 14.
Which is like 2x1, 2x2, 2x3,2x4,2x5,2x6,2x7, etc, etc.