Glad you asked!
When you submit your work to our novel contest, you automatically have 100 free copies that people can reserve to put on their reading lists. Every time a copy is "reserved," the number of available copies of your work goes down. This is both to limit the number of free copies circulating the internet, and to give you, the author, an indication of how many people are reading your work!
When the number of copies available falls below ten, you have the option to add 20 more. This is completely optional, but can be a good idea if you want more people to be able to read your book (the more reader engagement data you have, the closer you are to winning the contest).