By default, moving average values are put at the period in which they are calculated. For example, for a moving average length of 3, the first numeric moving average value is put at period 3, the next at period 4, and so on.
When you center the moving averages, they are put at the center of the range instead of at the end of it. This is done to put the moving average values at their central positions in time.
Suppose the moving average length is 3. In this case, Minitab puts the first numeric moving average value at period 2, the next at period 3, and so on. In this case, the moving average value for the first and last periods is missing (*).
Suppose the moving average length is 4. The center of that range is 2.5, but you cannot put a moving average value at period 2.5. This is how Minitab works around the problem. Calculate the average of the first four values, call it MA1. Calculate the average of the next four values, call it MA2. Average those two numbers (MA1 and MA2), and put that value at period 3. Repeat throughout the series. In this case, the moving average values for the first two and last two periods are missing (*).