The term “selective service” is typically used in the United States to refer to drafting people into the military. The Selective Service Act of 1917, then, was a law that set up a military draft in that year.
In 1917, the United States entered World War I. However, its standing army was tiny and the number of volunteers that it got upon declaring war was not big enough to create an army that would be big enough to make a difference in the war. For this reason, President Wilson asked for a draft to be instituted. Congress complied by passing this law. The law provided for the first military draft in the United States since the Civil War.