Statistical process on control (SPC) is a tool to control and monitor a process. To control the process important process parameters must be measured and analyzed. With SPC the goal is to detect when a process is out of control to minimize the amount of wrong products.
A point falls outside the UCL or LCL (3 sigma lines).
Two out of three successive points are outside the 2 sigma line (second gray line under or above the mean).
Four out of five successive points are outside the 1 sigma line (first gray line under or above the mean).
Eight successive points are on one side of the mean line.
When one of these rules happens it will be visible as a red dot in the plot. If needed corrective actions must be taken to restore the process. Keep in mind a point outside the ULC and LCL will can occur in about 1 in the 371 data-points.