First of all, I don't know your math foundation.

If it is a primary school, it is the result of adding two natural numbers.

If it is middle school mathematics, then sum refers to the result of adding each number or algebraic expression.

If it is advanced mathematics, the meaning of summation is different, because the operations of middle school mathematics are all in Euclidean space, and there are non-Euclidean spaces outside Euclidean space, in which the addition operation will be redefined, and the natural summation will be different.