Image Histogram - What Is the Interpretation of PDF
Well, If you model your image as a realization of a random variable generator then the Histogram is the best estimation (Assuming no other information like prior, etc..) you have for the PDF of the random variable.  

For instance, you can see this model is used when doing Histogram Equalization (Transforming the realization into a realization of Uniformity Generator).

Pay attention that this is a very simple model.
For instance it doesn't take care of the correlation between adjacent pixels in image.  

Indeed your interpretation is correct given the model.