2nd derivative, convex, concave
The sign of the 2nd derivative indicates the curvature of the function.
The 2nd derivative is positive f''(x)>0 when the function is convex (open upwards) and negative f''(x)<0 the function is concave (open downwards). It is zero at the inflection points.
The secant lines are above the function graph for convex intervals and below the graph for concave intervals.
Move the point B along the function to see the values of the 2nd derivative f''(x).