What does it mean to delete your iOS update?

IIm currently struggling to keep up a 16GB iPhone 6 storage. It's always telling me to manage my storage and how I can't update apps because of it. I go to Settings and Manage Storage and the IOS 9.3.1 is Third place when it comes to most used up. It gives me an option to delete this update, but what does this mean? Will be apps be deleted? Please let me know!

Check and see what's using up most of your space