Meaning 1 ►▼
A ball hit hard up the middle might score a hit, the batter reaching base safely. It also might help help a runner on 3rd score a run.
Derek Jeter hit a ball hard up the middle.
Meaning 2 ►▼
being aggressive rather than "cute or clever"
“This wasn’t the time for compromise. With Trump and Cruz fighting for the nomination, Democrats and Obama should drive the ball hard up the middle and quit trying to be cute and clever.“ Principia in St. Louis