In simple terms, personification is a way of describing something that isn't human as human. It is a common feature of literary works and adds richness and depth to the written language. It is especially common in poems, endowing all kinds of natural features of the universe such as mountains, trees, and the moon with human characteristics.
In the first five lines of Carl Sandburg's "Chicago," we have a number of examples of personification. For instance, the first line refers to the city as "Hog Butcher for the World." The city itself isn't a butcher, of course; Sandburg is referring to Chicago's reputation at that time as the epicenter of the meat-packing industry. Similar personification is in evidence in the next two lines:
Tool Maker, Stacker of Wheat,Player with Railroads and the Nation's Freight Handler;
Once again, Sandburg uses personification to highlight certain aspects of the city's economic life. As well as being renowned for its stockyards, Chicago was a thriving railroad hub, not to mention a center of the manufacture of tools and the trading of wheat. This is truly a bustling metropolis whose whole personality is bound up with its economic activity.
But more than that, Chicago is a fiercely proud and independent city, whose spirit of toughness and endurance is personified in Sandburg's reference to "Stormy, Husking, Brawling/City of the Big Shoulders."