proof by mathematical induction with the summation operator?

$$
\sum_{k=1}^n k^3 = \left( \sum_{k=1}^n k \right)^2
$$
I can't quite understand this expression, and in fact this is my biggest difficulty in finding a solution. Can someone please explain to me ?
$$
\sum_{k=1}^n k = \frac{n(n+1)} 2
$$