In an information sense we remove attributes that identify the special cases retaining the attributes that describe the class of things which contain the special cases. Another way to say what generalization is, is to ask what do all the things in a group have in common.
Here is a thought exercise. Imagine your favorite tree. Mine is a willow tree in our back yard when I was 5. Now imagine another tree. Mine is a maple in my back yard today. Now think of them together. I can't really superimpose them, though they are both large. And there it was, the leap. I categorized the trees by size. I set aside interesting differences and collected the similarities.
This prioritization of properties or attributes is generalization. We are discovering treeness or what it is to be a tree. Trees have a characteristic trunk. Trees are bigger than bushes generally, though there is some confusing overlap.
People do this implicitly. In the worst case it is called prejudice. One of the better forms is called discernment.
A sloppy way to think of it, is to put things in a group and give it a label. Prejudice is when we too hastily place something in the wrong group.
A systems engineer must carefully choose his attributes and quantify their value ranges. Getting the attributes wrong will cause the product to miss markets. Getting the values of the attributes wrong may result in a design that is impossible to reify.