What does it mean when there is nothing on top of sigma?

The upper limit of summation is usually omitted when the index is obvious from the context.

Example: If we know is an infinite sequence (i.e. a function , for instance), then we can denote the summation of all the terms in the sequence by



or by

20000 as first it increases by 2500 then by 5000 so next it will increase by 10000