De Brouckere mean diameter is a fancy way of measuring how big things are when they are all different sizes. Imagine you have a bunch of different balls - some are big, some are medium, and some are small. De Brouckere mean diameter helps you figure out what the average size of all those balls is.
Now, let's pretend we have a bunch of marbles. We can measure the diameter (how wide they are across) of each marble, and then we add up all these diameters. After we add them all up, we divide that number by the total amount of marbles we have. This gives us the average diameter of all the marbles according to de Brouckere mean diameter.
So, to summarize, de Brouckere mean diameter helps us find the average size of different things that are all different sizes. It does this by measuring the diameter of each thing, adding up all these diameters, and then dividing it by the total amount of things.