Why would you want to be able to focus and defocus?
The problem probably appears at school, but then you might not know what is meant with focus and defocus. Others can do it easily, but they forget to tell you how to do it. You might catch up later when there is no energetic blockage on your ability of focusing and defocussing. If there is a blockage, then that could prevent you from catching up. Focusing and defocussing are important skills as they give you more information on the object of attention. Looking from a wide angle places objects in perspective, you see where the main items are located. Then per noticed object you dive into the details. Then when you look globally you know which details belong to the objects, although you might not notice them at once.
In analogy, when you know how to focus and defocus you know how to study things. First you put the main objects in perspective through the content table. You might glance through the chapters. You know what is coming and where it is placed. You create a coherent view of the material to study.
You can dive into a chapter and learn about its contents in detail. Then you place the details in perspective by looking at the contents. The details are easier located from memory easier and your intelligence can combine them into structured information. You catch the idea, although you might need to go into detail.
When you know how to focus and defocus you can transfer that ability to your study. Facts and ideas become structured knowledge, the same as the object of attention contains details that form the object.
In quite some circumstances you need this ability. For instance, you need it when you travel, when you put an object together from a sketched drawing, when you cook. Life becomes easier when you know how to focus and defocus and probably school results will improve.