Not quite, for example, the function y=x^4 has y''(0)=0, but 0 is NOT a point of inflexion of y=x^4, as y=x^4 just looks like a parabola.
The key is that concavity must CHANGE on either side of a point of inflexion. Ie, we are looking for maxima and minima of y' rather than just looking for...