What was the Selective Service Act?
The Selective Service Act of 1917 enabled the US to raise a national army following the US entry into World War I.
On April 6th, 1917, the US Congress voted to declare war on Germany, and entered World War I following that. At the time, the US army was very small compared to the armies already active in the war, numbering roughly 121,000. For a comparison, during the battle of Verdun (February - December 1916) alone, the French army had mobilized 1.14 million troops, and between 156,000 and 163,000 French troops were killed in action.
By the guidelines set down in the act, all men aged 21 - 30 (later extended to 45) years of age had to register for the draft. By the end of World War I in November 1918, roughly 2.8 million American men had been drafted, which when added to the roughly 2 million volunteers that had joined the armed forces in that period, resulted in a total troop count of roughly 4.8 million - much higher than the pre-war count.
As a note, one of the defining factors that led to the armistice at the end of World War I was the entry of the US into the war. The influx of troops and manpower onto the struggling Allied side of the war helped to increase pressure on the equally struggling Central Powers, resulting in an eventual armistice.