So, imagine you have a toy box and you want to make a list of your favorite toys. The U.S. government has a list like that, but instead of toys, they list the companies that they like to hire to do work for them. These companies are called contractors.
Now, the U.S. government has a lot of work that needs to be done, like building roads, bridges, or even spaceships. But the government can't do all the work by themselves, so they hire contractors to help them.
The top 100 contractors are the companies that do the most work for the government. They are like the best toy on your toy list. They get a lot of money from the government to do the work they are good at.
These companies are chosen by the government because they are good at what they do and they can get the job done. The government wants to make sure that the work is done on time and on budget, and that the contractors are doing a good job, so they keep a list of the top 100 contractors.
So, to sum it up, the top 100 contractors of the U.S. federal government are the companies that are good at doing work for the government and get a lot of money for it.