From Ohio History Central
These Hundred Days' Men were to serve as garrison troops throughout the North and in areas of the South that Northern armies had occupied. After Confederate General John Hunt Morgan's raid into Ohio in 1863, Brough was especially concerned with preventing Confederate invasions of the North. The Hundred Days' Men would allow regular soldiers currently serving as garrison troops to be sent to advancing Union armies. In this way, the Union might win the war more quickly, hopefully in one hundred or fewer days.
The governors of these five states submitted their suggestion to Secretary of War Edwin Stanton, who placed the proposal before President Abraham Lincoln. Lincoln immediately approved the plan. Ohio easily provided the federal government with its share of the Hundred Days' Men. Nearly thirty-six thousand Ohio men actually reported for duty, exceeding Ohio's requirement by six thousand soldiers. The other states that agreed to enlist One Hundred Days' Men fell short in their pledges. The War Department accepted all of Ohio's recruits, and the men were ready for duty within sixteen days of enlistment. These men did help the Northern war effort, but the larger armies failed to accomplish their primary
objective -- defeating the South within one hundred days.
The American Civil War did not end until the spring of 1865.