What does it mean for a process to be stable? I have attached text to better exp

Question

What does it mean for a process to be stable?

I have attached text to better explain the question and so there is no confusion. Please answer in your own words so I can understand.

Process Stability

Although every process has variability and, thus, every unit produced is slightly different, a large number of the units when combined create a distribution for which the mean and standard deviation can be computed. The mean and standard deviation are important parameters for monitoring a process because they describe the overall behavior of the process and how well it compares to requirements. Besides for use in constructing control charts, they enable us to determine, for instance, that only 8 parts per million will be defective.

But the mean and standard deviation of a process can be determined only if the process is stable. A stable process is one where fundamental features of the process are repetitive and unchanging. In a stable process the mean and standard deviation are unchanging, that is, they will remain the same until something happens in the process that changes them. In the production of circular rods, for example, the average diameter of the rods produced will be the same today as it was yesterday—given that nothing has happened to the process to move the average up or down. The same applies to the standard deviation of the diameter. Even though the diameters of individual rods will vary slightly, in a stable process the standard deviation of the rods remains the same. In contrast, in an unstable process the mean and/or standard deviation of the process are constantly changing due to changes in factors of production such as workers, equipment settings, process procedures, or material qualities. As long as they keep changing, it is impossible for the process to produce output that is on average constant and uniform. The point is, unless a process is stable, the process mean and variability cannot be determined, and the process cannot be monitored for quality.

Thus, first things first. Before employing control charts, it is important first to ensure that the process is stable. If it is not, the sources of instability (procedures, equipment, labor, material, worker skills, etc.) must be identified and remedied.

Explanation / Answer

Each and every process even if it is human operated or machine operated will have some form of defect or variance from its mean values. This variance is what causes the difference in the analysis of the required quantity and the present quantity. This variation can be factored into the processes from the start and can be implemented in such a way that no matter the variation, the system accounts for it and creates a stable process. This essentially means that when a process is stable, any variation that is recorded can be effectively measured, calculated and dealt with accordingly. The target value will always be close to the actual value with minimal to no variation recorded. This is what we call a stable system or a stable process.

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