As a result of the fall of Forts Henry and Donelson, Confederate Gen. Albert Sidney Johnston, the commander in the area, was forced to fall back, giving up Kentucky and much of West and Middle Tennessee. He chose Corinth, Mississippi, a major transportation center, as the staging area for an offensive against Maj. Gen. Ulysses S. Grant and his Army of the Tennessee before the Army of the Ohio, under Maj. Gen. Don Carlos Buell, could join it. The Confederate retrenchment was a surprise, although a pleasant one, to the Union forces, and it took Grant, with about 40,000 men, some time to mount a southern offensive, along the Tennessee River, toward Pittsburg Landing.
More details on the battle of Shiloh are available here.