It would be the percentage of Unique visitors that have seen the specific page for the specific month.
0.0% would mean that here is even less than 0.1% of Unique visitors that have seen these pages for the given month (i.e maybe 0.01)
You might want to run the unique visitor metric report by month and see what is the total of unique visitors and do the calculation manually to see the percentage you see.
Notice that the total is 0.1% and it does not correspond to the sum of the percentages, this is simply due that the total is always deduplicated.