The curvature is the second derivative of the function at the minimum.

At a typical minimum, where the function has vanishing first derivative, the second derivative will be a positive number whose magnitude characterizes the "steepness" with which the function increases as one moves away from the minimum.

At an atypical minimum, for example an interval boundary or a non-smooth function, this value may be meaningless.

Even in the case of a typical minimum, the value of the curvature property will typically be accurate only to a handfull of digits. If you require a highly accurate determination of the curvature, you should compute the second derivative of the minimzed function explicitly.