I **presume** you are referring to when you look at the Settings App, General, About, and scroll down ... would be helpful to pretend we're all blind and can't see what you see (because we are).
The three dots mean the number is truncated because you don't have a large enough screen to show the entire number (which is 20 digits long), I believe. I can't be sure because my iPhone 6s is large enough to display the entire number, and I don't have my smaller iPhone models (a 5s and 4) nearby where I am at the moment.