The symbol $\sum_{1\le k\le n}$ is a bit unusual, but in the present context it means that we take the sum over thes set of pairs of integers $P = \{(k,n) \in \mathbb N \times \mathbb N \mid 1 \le k \le n \}$.
But how is the value of a "series" of the form $\sum_{(k.n) \in P} u_{k,n}$ defined? We say that $\sum_{(k,n) \in P} u_{k,n} = u$ if for all $\epsilon > 0$ there exists a finite subset $P_\epsilon \subset P$ such that for all finite subsets $P' \subset P$ with $P_\epsilon \subset P'$
$$\left\lvert \sum_{(k,n) \in P'} u_{k,n} - u \right\rvert <\epsilon .$$
Of course it requires a proof that the first equation is true (we have to compare various "infinite sums"). However, it is obvious that each single summand $\frac{1}{n(n+1)} k a_k$ oocurs on both sides of the equation. The issue is to prove that the limit of the RHS exists and equals the LHS. I shall not do that here.