This has to do with not getting off the subject, not bringing in material that is irrelevant. You might think of this in terms of focusing a camera: you want to get a picture of something in particular, with perhaps a little background or context, and you want the details to be sharp. Problems with focus often originate in the planning stage of writing, so if you have trouble with this you might try outlining or taking notes to clarify for yourself what you should include.