Pittsburgh-Des Moines Steel Co. was a company that made things out of steel, like big buildings and bridges. They would take raw steel and use big machines to shape it into different parts that they could use to build all kinds of things.
For example, if they were building a big bridge, they would use steel beams to make the structure strong enough to hold all the weight of cars and trucks driving over it. They would also use steel cables to support the bridge and keep it from falling down.
Pittsburgh-Des Moines Steel Co. was really good at making all kinds of steel products, which is why they were so successful for many years. However, eventually, they went out of business for a variety of reasons, like competition from other companies and changes in the industry.