The center line on a U chart represents the average number of defects per unit of measurement. The average number of defects per unit is also called the process mean.
Minitab Express displays the mean in the output pane and indicates whether the value is estimated from the data or is specified.
Use the center line to observe how the process performs compared to the average. If the process is in control, then the points vary randomly around the center line.
Do not confuse the center line with the target value for your process. The target is your desired outcome. The center line is the actual outcome.