That means the amount of space given to a topic or an event. The more space in relation to the total work, the more important that which is being shared by the author is. For example the 21 chapters of the gospel of John cover about 3.5 years of Jesus’ life. Chapters 13 – 17 cover about 6 hours. That is about .096% of Jesus’ life yet John devotes 23.8% of his gospel to that six hours. That might suggest it is important. You would need to validate that importance during your study, but the sheer weight of the proportion calls attention to that section of the gospel.
More to look for in later posts.