This is a baseball term defined as an easy pitch to hit, thrown right down the middle of the plate. It does not matter what type of pitch, or necessarily how fast it is thrown, only that it is an easy pitch to hit. These types of pitches are never thrown on purpose in a major league game. A good example of a meatball pitch is the pitches thrown to players during the Home Run Derby which takes place during the All-Star break.
An easy pitch is known as a meatball because of the similarities in shape between a baseball and a meatball, and the presumed delicious meatball is just as enticing as an easy pitch to hit. There is no reference as to when the phrase started being used, but it has been used as far back as the 1940's. An example of usage is, "Billy knocked that meatball right out of the park!"