What conditions allow vectors $u$ and $v$ to be true in ${ℝ}^2$ and ${ℝ}^3$

Question

I am having trouble understanding what this problem in my book is asking.

It says "Under what conditions is the following true for vectors $u$ and $v$ in ${ℝ}^2$ and ${ℝ}^3$"

and then it gives this equation $||u+v|| = ||u|| + ||v||$

Can someone explain what conditions it is referring to or point me in the right direction?