Up until 1941, public support for the United States joining the war was very tepid. Even after the attack on Pearl Harbor, the government had to work hard to convince people to buy war bonds that could be used to fund the war.
Government promotional materials like these posters were designed to convince the public that everyone was working together, doing their part to win the war. Those employed at home and not serving in the military were asked to "do their part" by buying war bonds. These bonds were loans to the government. People bought the bonds with the promise that the government pay them back with interest after ten years or more.
During the war, the government needed all the extra money it could get to help pay for war equipment. War bonds or defense bonds were bought by purchasing stamps for 10 or 15 cents each. The stamps were then pasted into booklets that could be turned in when full for a $25 war bond.
War stamps and bonds were not only a way for the government to raise money, but a way for every person to participate in the war effort. It became very patriotic to buy bonds and stamps.