That is a constructor. It's what's known as a "default constructor", i.e. it intializes all the values to some pre-determined initial value, in this case -1 and "unknown".
As a second note, if you are going to be providing other constructors that create an initial value (say, a constructor which also sets the city name), I would recommend calling that constructor from the the default constructor. This reduces the amount of code you have to check for problems.
// call the other constructor that actually assigns values
public City(String name, int population)
this.name = name;
this.population = population;