The key feature that helps to clearly illustrate that the graph of \( f(x) = x^2 - 3 \) has been moved down 3 units from the parent function \( y = x^2 \) is the y-intercept.
The y-intercept of the original parent function \( y = x^2 \) is at (0, 0), while the y-intercept of the function \( f(x) = x^2 - 3 \) is at (0, -3), clearly indicating that the graph has been shifted down 3 units.