question from do carmo diff. geometry I am studying differential geometry myself from Do carmo and i didn't understand the question :

show that if a surface is tangent to a plane along a curve , then the points of this curve are either parabolic or planar .

At the question i didn't understand the sentence ' surface is tangent to a plane along a curve ' 
Please firstly help  me about it and later maybe hints for solution   

That a surface $S$ is tangent to a given plane $\pi$ along a curve $\gamma$ means that the tangent plane of $S$ at points of $\gamma$ is the given plane $\pi$. So the tangent plane of $S$ is the same along $\gamma$. Then the derivative of a unit normal vector field of $S$ along $\gamma$ is zero. So the shape operator of $S$ at points of $\gamma$ has $\gamma'$ in its kernel. Finally, keep in mind that parabolic or planar points are points of $S$ where the shape operator has kernel i.e. points where the Gauss curvature vanish.
