I've made some slight changes to the description.
Basically, the problem is a slight variant of the stock exercise that is used
to familiarize the learner with the for loop. You basically need to assign an
appropriate sign to each of the numbers in the array or list.

Method type should be "bool" not "Boolean"

Btw, that's not an issue, but a question. That being said, the text you've quoted shouldn't give you any troubles. If you don't know lists (or arrays) yet, this kata isn't for you. If you don't know what abs means, well, it returns the absolute value of a number.

"sum_of_n (or SequenceSum.sumOfN in Java, SequenceSum.SumOfN in C#) takes an integer n and returns a List (an Array in Java/C#) of length abs(n) + 1."

Pretty much the whole thing but the above really threw me! I think it's me just getting freaked out by all of the maths involved.

What part of the problem did you not understand?

I struggled to understand the instructions.