“Drop off” reflects the page on which visitor left an identified “Visitors Flow” – the intended flow of your traffic – and went down a path different than the Visitors Flow. In taking this action, the visitor either clicked a link on your website that doesn’t correspond with your Visitors Flow or they left your site altogether. The point is that they diverged from the “Visitors Path” as visualized and presented in GA (goals).
The takeaway from this detail is that a “drop off” indicate where your Visitor’s Flow has holes through which people are “dropping off”. The business value depends on how the site content is arranged. You may have some content on a page that visitors are not reaching because they exit the site before reaching it.
There is no set number, but consider common sense when determining if too many visitors are leaving the page. A small percentage is an inevitable, but if a large percentage is dropping off, the content or pages should be rearranged to make important information more prominent and more visible.