You've heard the term. What does it mean?
Whether it's tee work, soft toss, front flips, or batting practice, there should be a purpose behind each swing. Staying inside of the ball, literally trying to hit the inner half of the baseball, is a discipline that will help a hitter use the middle of the field rather than being more prone to "rolling over" the ball. Roll over is a term used when the barrel travels too far in front of the hands.
A hit and run for a right-handed hitter that is properly executed forces the hitter to stay inside of the ball. Typically the second baseman will be covering second base on a steal. The hitter must put the ball in play, preferably on the ground and to the right side (second base). If the hitter gets around the ball ("rolls over") then it's very difficult to hit the ball to the right side of the field.