An ellipsis (dot-dot-dot) is used to show something is intentionally missing or to show when someone is thinking when talking. (uhh...)
Say I have a quotation like this one.
Today, the city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation, with O'Hare International Airport being the busiest airport in the world; it also has the largest number of U.S. highways and railroad freight entering its region.
Perhaps I want to leave out the part about the airport. I could use an ellipsis to show that it's intentionally missing.
Today, the city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation...;it also has the largest number of U.S. highways and railroad freight entering its region.