The Bonus Army was a group of veterans who had been in the Army during World War I. They had been promised a bonus -- some extra money -- that was going to be paid to them in 1941.
As the depression went along, things got to be very hard for many people, including some veterans. Because of this, the Bonus Army organized to go to Washington DC to ask if they could have their bonus right away. The idea was that it could help them much more now when they needed it than it would in 10 years or so.
Congress refused to give out the bonus and soldiers were brought out to help drive the Bonus Army out of the city.