In mathematics, it means sum.

K in your formula is a summation index, and the whole formula means that k takes an integer from 2 to n.

For example, ∑(k), the subscripts are n and 2 respectively, indicating 2+3+4+5 … n.

I'm new to this high school, so I don't usually talk about it specifically. I understand it according to the meaning of the question. I'll know if I do more. It's just an expression, and primary schools can actually understand it, but primary schools generally encounter such problems.