Walter Shewhart, who developed Control Charts at Bell Labs in the 1920s, used those charts to distinguish between 2 types of variation. He saw that all processes showed signs of Common Cause variation, but processes that were not in control also exhibited Special Cause variation.
If you think about why you’re maybe a few minutes early or a few minutes late getting to work each day, you might say it was a combination of the traffic, timing of lights, the line at Starbucks, etc. This is “Common Cause” variation. But if one day you were 3 hours late, it might be due to a major accident, or because your car broke down. In these examples, the cause of the variation would be “assignable” which is “Special Cause.”
So Common Cause variation is random, frequent and generally present in the process whereas Special Cause variation is rare and due to a specific event. In order to control special cause variation you would deal with the specific event but in order to reduce common cause variation in a process, you would initiate a Lean Six Sigma project.