As an illustration of abstraction, think of the maps that you might find in a geographical atlas. There are various different types of maps. At each level, certain information is included, and certain information is omitted. A map of the world might only contain the names of vary large features, such as mountain ranges, or large political boundaries. A map of a contenent can contain more information, such as all political boundaries, and maybe the names of large cities. Moving to a smaller area, a map of a country would contain more cities, and maybe start to contain information on major roads or highways. A map of a large city might contain information on all roads, and perhaps major structures of importance. A map of just a portion of a city will often contain the most information of all, indicating individual buildings and perhaps their occupants.
At each level the developers have selected an approprate degree of abstraction, including certain information and omitting other information.