The mathematical symbol you framed is the summation symbol ∑, which is pronounced "sigma". This symbol is usually used to represent the sum of each element in a sequence or set. Its mathematical expression is:

∑ (starting from the first natural number n less than or equal to k) to the function value f(n) of all natural numbers n of k.

It means that all the elements in a sequence (finite or infinite) whose subscripts start with n are added together, and the value obtained is the value of this symbol. For example, if there is a sequence a = {1, 2,3,4,5}, then ∑a means 1+2+3+4+5= 15.