This is a baseball reference to the second baseman and shortstop, whose positions lie in the middle of the infield. More than any other two infield pairs, the middle infielders must have great communication and ability. Both positions require speed, agility, and the ability to make accurate throws. The shortstop must typically be the best defensive player on the team, because they are responsible for covering a large portion of the field in an area where a lot of balls get hit. Any ball hit between third base and second base are the responsibility of the shortstop. The second baseman covers the area between first and second.
The phrase "middle infielders" is typically used in discussion about players and teams. Since both players have similar skill sets, it is possible for a player to be moved from one position to the other. However, since they are both very different positions, this rarely happens. Even though their standard positions exist on either side of second base, there will be instances when they will shift off to one side or the other. There will be instances when the shortstop and second baseman are not responsible for defense in the middle of the field.