Resposta
$\vec{u} \cdot \vec{v} = x_1 x_2 + y_1 y_2 = |\vec{u}||\vec{v}|\cos\theta$; $\vec{u} \cdot \vec{v} = 0$ indicates that the vectors are orthogonal (perpendicular).
Explicação
Because $\cos(90^\circ) = 0$, non-zero vectors with a zero dot product meet at a right angle.